THE MECHANISM OF HEREDITYAND EVOLUTION.

By C. C. HURST, Ph.D. (CaIitab.), F.L.S.

Abstract of Paper read before The Eugenics Society, 23rd March, 1927).

Gregor Mendel was the first to conceive a mechanism of heredity'in his experiments with Peas published in 1866. UnfortunatelyMendel's important paper remained unknown to Scieice until it was,discovered by Hugo de Vries in 1900. Several authors, from Hippo--crates to Herbert Spencer, had previously suggested the general idea*of vital particles in an organism, but it was Charles Darwin who in1868 provided a sound basis for the later fruitful investiga-tions and theories of the mechanism of heredity, in his comprehensiveconception of Pangenesis. In the theory of Pangenesis (modestlyannounced as a provisional hypothesis) Darwin suggested a mechanismof heredity based on the existence of minute representative livinggemmules in the cells. This conception was of vital importance, sinceit led directly to the work of Galton, de Vries, Weismann, the discovery,of Mendel, the experiments of Bateson and others, and to theexperimental discoveries of Morgan and his colleagues. To-day,after more than half a century of revolutionary experimental investiga-tions in heredity, Darwin's conception of represexitative living units*in the cells, remains the fundamental principle of the mechanism ofheredity. Darwin's speculative gemmules of the nineteenth centuryhave become the experimentally demonstrated genes of the twentieth,century, and the reality of their existence in the cell is now as certainas that of the electrons in the atom.

The fundamental importance of the gene in biology is certainlyas great as that of the electron in physics and the atom in chemistry.

Francis Galton, the founder of the philosophy of Eugenics, after',considerable criticism and modification, was the first to accept Darwin' stheory of Pangenesis in 1875. The discovery of Hertwig in the sameyear, and of Van Beneden in 1883 that the nucleus of the cell is theactual vehicle of heredity, cleared the way for the important develop-ment of Darwin's concept of Pangenesis by de Vries in 1889 in his

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theory of Intracellular Pangenesis which placed Darwin's originaltheory on the firm footing of the observed facts at that date. In 1892Weismann, accepting Darwin' s "ideal theory" of Pangenesis asmodified by Galton and de Vries, demonstrated the importance of thechromosomes as the carriers of the units of heredity. The discoveryof Mendel's long lost paper in 1900 heralded a new epoch in the historyof heredity, since it provided an experimental demonstration of theexistence of the units of heredity postulated by Darwin and his succes-sors. In 1902 and later, Bateson and others extended Mendel' sgenetical experiments with Peas to all kinds of plants and animnals.In r9IQJIbrgan.published.the first results of his genetical experimentswith the- Fruit Fly Drosophila, culminating in 1925 with the convincing-monograph on the Genetics of Drosophila which experimentallydemonstrated the mechanism of the genes in heredity. Morgan andothers have established the relative locations of many genes, in each ofthe four chromosomes of the Fruit Fly Drosophila melanogaster, andhave demonstrated a linkage system of these genes within each chromo-some. So far about 400 genes have been located in this species, ofwhich about 150 are in the sex chromosome so that the characters.which they represent are sex-linked in their inheritance. Morgan'sresults in Drosophila have already been confirmed in other genera ofanimals and plants and are being rapidly extended.

As a result of this work it is now experimentally demonstratedthat the genes representing the characters of plants and animals arecarried in the chromosomes of the nucleus of each cell. Consequentlythe mechanism of the chromosomes in development and in the formationof the germ cells has a fundamental significance in heredity and evolu-.tion. In other -words the mechanism of heredity and evolution is themechanism of the chromosomes and the genes therein.

Mechaniam of Development.Chromosomes are remarkably similar in appearance in plants and

animals. 'If a group of cells be taken from a plant and compared witha group of cells from an animal each cell will be found to have a centralnucleus, and in the cells about to increase by division, thechromosomes appear in the nuclei as rod-like bodies which dividelongitudinally, one half of each chromosome going to one pole of thecell and the other half to the other pole, eventually making two cellsout of one. The two new cells formed contain the same number ofchromosomes as the original mother cell. This mechanism of develop-ment which is known as Mitosis, is carried through by the chromosomes.in every somatic division of the multitudinous cells of plants andanimals as growth proceeds.

The remarkable technique developed by Chambers in his tissuecultures shows that the living chromosome is a gelatinous extensibleand contractible body, with a definite structure of granules arranged inrows about a non-granular core. Wenrich took 18 chromosomes of theB pair from 13 individuals of the Grasshopper (Phrynotettix magnus)and these showed a striking constancy in size and arrangement of theprincipal chromatic granules or chromomeres,and the same constancywas shown in the different cells of a single individual.

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THE MECHANISM OF HEREDITY AND EVOLUTION. 21

On the other hand he found that different chromosomes have,different arrangements of chromomeres. This is visible evidence that-the chromosome is a compound body with a definite longitudinaldifferentiation. In view of these visible indications of structuraldifferentiation in the chromosome it is not difficult to appreciate theexistence of more minute structures beyond the visible chromomeresinto the invisible ranges of the gene.

Alechanism of Heredity.In the formation of the germ cells quite a different mechanism of

the chromosomes is seen both in plants and animals. At a certainstage known as the maturation or reduction division, both the maleand female germ cells in plants and animals reduce the number of theirchromosomes by one half. For instance, to take two extreme cases,the Nematode Threadworm (Ascaris megalocephala univalens) hasonly two chromosomes in its germ-cells before maturation. At thereduction division one of these chromosomes goes to one pole and theother chromosome to the other, so that the eggs and sperms containonly one chromosome. Similarly in the Crayfish (Cambarus virilis)which has 200 chromosomes before maturation, at the reduction division100 chromosomes go to one pole and 100 chromosomes to the other pole.This mechanism of heredity is known as Meiosis, and is carried throughby the chromosomes in every reduction division in the formation ofeggs and sperms in both animals and plants. As Weismann foresaw,in 1892, this reduction mechanism of the chromosomes is of fundamental-significance, for if the chromosomes segregate at random in the reduc-tion division, and each chromosome carries a different complex ofgenes, then the possibilities of variation in the eggs and sperms aremultitudinous. That the chromosomes do actually segregate atrandom in the reduction division has been fully demonstrated by MissCarothers in several genera of Grasshoppers (Orthoptera) and also byother observers in other plants and animals.

This qualitative reduction of the chromosomes in eggs and spermsis the mechanism of Mendelian heredity, and provides a completeexplanation of the Mendelian ratios in independent assortments.

The quantitative reduction of the chromosomes in eggs andsperms is also one of the essentials of the process of fertilisation, since-the union of the reduced maternal and paternal chromosomes in theact of fertilisation provides the normal number of chromosomes for the-embryo plant and animal,and enables its development to proceed onnormal lines, so that each cell in the body contains an equal number ofmaternal and paternal chromosomes.

Sex Chromosomes.In many animals and plants with separate sexes the male and

female sexes are assbciated with the presence or absence of certainchromosomes in the nuclei of their cells. For instance, the hemipteraninsect Protenor has 13 chromosomes in the male and 14 chromosomesin the female. The largest chromosome is the sex chromosome, themale having one sex chromosome (X) and the female two (XX). In thehemipteran insect Lygaeus there are 14 chromosomes in the male and fe-

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male, but in this case the smallest chromosomes are the sex chromosomes,.the female has two sex chromosomes of equal size while the male has.two sex chromosomes which are unequal in size. Consequently afterreduction the sperms are of two kinds, carrying X or Y, while the eggsare all of one kind carrying an X chromosome. Fertilisation thereforeproduces on the average equal numbers of males XY and females XX.

In the flowering plants with sexes on separate plants (DioeciousAngiosperms) the Tape Grass (Vallisneria) has 17 chromosomes in themale plant and 18 in the female. The Water Thyme (Elodea) has 48chromosomes in each sex, the male plant having XY chromosomes andthe female XX. Other plants and animals have similar mechanismsof sex chromosomes with variations. In the Sorrel Plant (Rumex), forinstance, the X chromosome is represented by a large M chromosome,while the Y chromosome is apparently a compound of two small mmchromosomes. In the Nematode Threadworm (Ascaris megalo-cephala) there is in the male a single small X chromosome which is.usually attached to the end of one of the other chromosomes.

A number of insects, domesticated mammals and lizards belongto the XO type of sex chromosomes, while other insects and mammals,including monkeys and Man, belong to the XY typeof sexchromosomes.

Wilson reports that in the hemipteran insect (Metapodius) theY chromosome may be either present or absent, so that it is probablethat the XO type has been derived from the XY type through the loss.of the Y chromosome. In Lepidoptera (Moths) and Birds, the femaleproduces two kinds of germ cells rather than the male, the samemechanism is found but the sex chromosomes are known as ZW insteadof XY in order to distinguish the female digametic from the maledigametip organisms.

For instance in the Moth (Talaeporia tubulosa) Seiler found 59,chromosomes in the female and 60 in the male, with two kinds of eggscontaining 29 and 30 chromosomes respectively, hence the female isZO and the male is ZZ. In another Moth (Phragmatobia fuliginosa).Seiler found 56 chromosomes in both male and female, but the male isZZ with two like sex chromosomes, while the female is ZW with twounlike sex chromosomes.

A chromosome mechanism of sex of the XY type in the male hasbeen found in the lowly Liverwort (Sphaerocarpos) a Bryophyte relatedto the Ancient Mosses.

In Man all the recent evidence shows sex chromosomes of the XYor XO type, and Painter has demonstrated the XY type clearly in bothwhite and negro races. The value of a knowledge of the sex chromo-somes in Man to Eugenists is seen when such complicated sex linkedcharacters as Colour Blindness are investigated, for without a knowledgeof the mechanism of the sex chromosomes the observedfacts are puzzlingand paradoxical. All observations show that a colour blind fatherand a normal mather have all their sons and daughters normal. Allthe daughters can transmit the defect to future generations, but noneof the sons can do so. If the daughters marry normal men the defectwill be transmitted to one half of their children of both sexes, but onlythe sons will be colour blind. On the other hand if the daughtersmarry colour blind men, one half of their daughters and one half of

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THE MECHANISM OF HEREDITY AND EVOLUTION. 23

their sons will be colour blind. In such cases the affected sons anddaughters will carry on the defect, and if an affected daughter marries acolour blind man all their children, sons and daughters, will be colourblind. The unaffected daughters will still carry on the defect, but theunaffected sons will not. The explanation of all these complicatedfacts is to be found in the simple mechanism of the X and Y chromo-somes. The gene for colour blindness is located in the X chromosomeand not in the Y chromosome. If X represents the sex chromosomecarrying the gene for normal colour perception, and X represents thesex chromosome carrying the gene for colour blindness, then all malesare either XY normal or XY colour blind. Females on the other handare of three kinds XX inormal, XX normal, or XX colour blind, onedose of the gene makes a man colour blind, but it takes two doses of thegene to make a woman colour blind. The Mendelian segregation of theX X and Y chromosomes in the reduction divisions of the eggs andsperms and their random matings in fertilisation, provide a simpleexplanation of the complicated facts of the inheritance of colour blind-ness in Man.

Specific Sets of Chromosomes.

Recently Cytology with its modern refinements of technique hasmade remarkable progress in the analyses of chromosome complexes.So far, no less than 2,845 species of plants and animals, representing1,326 genera, 417 families, 181 orders, 77 classes and 33 phyla, havebeen examined.

The chromosonme numbers found in these species range from 1 pairin the Nematode Threadworm (Ascaris) to more than 100 pairs in theCrustacean Decapod (Cambarus) (Crayfish), while in plants the numbersrange from 2 pairs in the Fungi (Eumycetes) to more than 100 pairs inthe Horsetail (Equisetum) and the Ferns (Ophioglossum and Ceratopteris.In all recent cases where large numbers have been examined by severalobservers, it has been found the number of chromosomes or chromosomesets is constant and characteristic for each species.

It is now possible therefore to present a more precise conception of aspecies. A species is a group of individuals of common descent, withcertain constant characters in common which are represented in thenucleus of each cell by constant and characteristic sets of chromosomes.Specific sets of chromosomes may differ in number, size and shape ofchromosomes, structure, behaviour and gene content, the whole con-stituting a dynamic specific complex present in all the cells of all theindividuals of a species.

It is important, however, to emphasize the fact that visibly identicalchromosomes sets may be entirely different in their genetic constitu-tion, they may have similar genes in different combinations andarrangements or they may have entirely different genes. For thisreason chromosome numbers in certain species are only of secondaryimportance. Comparison of the chromosome sets of different groups ofplants and animals at corresponding stages often show wide differencesin number, size and shape of the individual chromosomes. On theother handin many cases there is a pronounced family likeness within

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the order or the family, as in the genera and species of OrthopteranGrasshoppers and the Dipteran Flies.

McClung and his students have examined more than 100 genera,including 800 species of the Acrididian shorthorned grasshoppers, andthroughout the family the males as a rule contain 23 chromosomesand the females 24, while the eggs have 12 chromosomes and the sperms11 or 12. This group of animals has existed for millions of years andonly an extremely precise mechanism could have preserved this com-mon series of chromosomes in all the multitudinous cells that haveexisted in this group through the ages.

The 12 pairs of chromosomes in this group represent a gradedseries of sizes from the smallest to the largest, andthe largest may be 10times larger than the smallest. Each genus of this family differs fromthe other in the size and form of its chromosomes, and here it is evidentthat the degree of relationship is as clearly expressed in the chromosomecomplex as in'the external characters of these genera, indicating adescent by modification from a common ancestral series of chromosomesparalleled by corresponding modifications in the bodily structures. Apractical demonstration of this was seen when McClung in 1917 dis-covered a new species of Mermiria based on a difference in the form ofone chromosome, which was in 1919 confirmed taxonomically by thesystematist Rehn. In an analysis of the chromosomes of 30 species ofFlies (Drosophilideae) Metz has reduced the different chromosome setsto 12 principal types differing from one another in number, size andform. These species have 3, 4, 5 or 6 pairs of chromosomes of differentsizes and shapes, and there is good genetic evidence to show that allthese types of chromosome sets have been derived from 1 or 2 originaltypes by segmentation, loss or fusion, and it is evident that the degreeof relationship between these species is equally manifest in the chromo-somes as in the characters, indicating descent from a common ancestralspecies by modifications of the chromosomes.

Similar specific differences in the chromosome sets are found inplants. In the Hawksbeard (Crepis) for instance, species are foundwith 3, 4 and 5 pairs of chromosomes.

Nawaschin has sorted out the chromosomes of 10 of these speciesinto 5 different shapes, and finds that A, C and D shapes occur in all the10 species, B shape is in all but 1 species, while E shape is found in 3species only. The same shaped chromosomes however differ much inlength, and presumably carry variable numbers and arrangements ofgenes.

The genus Carex the Sedge Plant is one of the most variable ofplant genera in its chromosome numbers.

Heilborn's work shows 44 species with 22 different chromosomenumbers varying from 9 to 56 pairs. In this genus the different num-bers and sizes of the chromosomes in each species clearly havetaxonomicsignificance, and Heilborn found that species belonging to the samesection of the genus have chromosome numbers of about the same height.

Blakeslee's genetic experiments with the Thorn apple or JimsonWeed (Datura Stramonium), show another way in which in such a genusas Carex the chromosome numbers may have increased one by onethrough the non-disjunction of a pair of chromosomes in the reduction

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THE MECHANISM OF HEREDITY AND EVOLUTION'. 25

'division. The normal number of chromosomes in this species ofDatura is12 pairs ofvariable forms and sizes. In Blakeslee' s experiments varietiesfrequently arose, through non-disjunction, with an extra chromosome,i.e., 25 instead of 24. In course of time Blakeslee succeeded in getting12 distinct heteroploid varieties of this species, each with an extrachromosome, thus demonstrating that each of the 12 chromosomescarried different genes. A similar case has been found in the EveningPrimrose (Oenothera Lamarckiana) by de Vries, Gates and others. Inthis species there are 7 pairs of chromosomes, and de Vries and Gateshave identified 7 distinct varieties of this species corresponding withthe 7 chromosomes.

The recent work of Clausen with Viola points to the probabilitythat some species in this genus have arisen in this way.

Polyploid Varieties.Various kinds of chromosome sets peculiar to different species,

genera and families of plants and animals have been reviewed,and ithas been noted that as a rule the number of chromosomes is constantand characteristic for each species. Recently a number of cases havebeen found in which individuals suddenly appeared with double thenumber of chromosomes characteristic of the species. These are thefourfold or tetraploid varieties, so called because the gametic sets ofchromosomes are fourfold in the somatic or body cells instead of thenormal twofold. One of the first tetraploid varieties in plants wasdiscovered by de Vries in the Evening Primrose (Oenothera Lamarckiana)and called 0. gigas. Later it was found to have 28 chromosomesinstead of the 14 usual in the species, consequently it may be regardedas a tetraploid variety of the diploid species. This tetraploidvarietyis larger in all its parts. Gates has made careful measurements ofvarious cells of the different tissues and finds them to vary from 1i to 4times larger. Later, triploid varieties of this species were found with21 chromosomes.

In Datura Stramonium, Blakeslee has found haploid, diploid,triploid and tetraploid varieties of this species, with 12, 24, 36 and 48chromosomes respectively. As all these 4 forms belong to the samespecies, the specific characters are the same, but varietally they aredistinct, and as the number of chromosomes increases the charactersbecome larger as a rule.

This increase in size following the doubling of the chromosomes isa marked feature in many tetraploid varieties that have arisen undercultivation, though not in all. Recently I have found 3 cases oftetraploid varieties in 3 diploid species of Rosa (R. macrophylla, R.cinnamomea and R. indica). In each case the wild diploid species has14 somatic and 7 gametic chromosomes, while the cultivated tetraploidvarieties have 28 somatic and 14 gametic chromosomes. The gianttetraploid varieties are indistinguishable specifically from the wilddiploid species, though varietally they differ in size and other minordetails. Several triploid varieties of R. indica with 21 somaticchromosomes have also been found. These cultivated polyploidvarieties with duplicated chromosomes should not be confused withthe wild polyploid species of Rosa with differential sets ofchromosomes,

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which are specifically distinct in their characters. Other polyploidvarieties with duplicated chromosomes are found in the Tomato(Solanum Lycopersicum), the Nightshade (Solanum nigrum), which wereartificially induced by grafting in Winkler' s experiments. It isinteresting to note that these two species of Solanum have a multipleseries of chromosome numbers, the Tomato having 12 gametic and 24somatic chromosomes, and the Nightshade 36 gametic and 72 somatic.The tetraploid varieties of each species have 48 and 144 somatic chro-mosomes respectively. The Iceland Poppy (Papaver nudicaule) has adecaploid variety with 70 somatic chromosomes, the diploid varietyhaving 7 gametic chromosomes. Ljungdahl has raised from these, bycrossing, a regular hexaploid variety, with 42 chromosomes, and aregular tetraploid variety with 28 chromosomes.

In animals polyploid varieties are well known in the NematodeThreadworm (Ascaris megalocephala). The diploid variety has 1gametic and 2 somatic chromosomes,while the tetraploid variety has 2gametic and 4 somatic chromosomes. Triploid, hexaploid and octo-ploid embryos have also been found in this species by Boveri and ZurStrassen.

In the Phyllopod Brine Shrimp (Artemia salina) there are twovarieties, one with 42 somatic chromosomes and the other with 84 asshown by Artom.

The diploid variety from Sardinia produces only sexual eggs, whilethe tetraploid variety from Capodistria produces only parthenogeneticeggs.

In the Fruit Fly (Drosophila melanogaster) there are diploidvarieties with 4 pairs of chromosomes, triploid varieties with 12chromosomes, and tetraploid varieties with 10 chromosomes. AsMorgan and Bridges have shown, the true triploid flies are femalesbecause they have 3 X chromosomes balanced against 3 of each kind ofordinary chromosomes. This is the same balance that produces thenormal diploid female which has 2 of each. Experiments show thatif there are only 2 X chromosomes present against 3 of the others,thefly is an intersex, while if only 1 X is present against 3 of the others,thefly is a supermale.

Similarly the tetraploid flies are female, as expected. All thisgoes to show that sex is determined by the reaction of the sex chromo-somes with the other chromosomes, and is not due, as was at firstthought, to the sex chromosome itself.

Po4yploid Species.The recent discovery of differential polyploidy seems likely to

throw considerable light on the problems and mechanism of evolution.Differential polyploidy was first recognised in 1923 in the wild speciesof the Dicotyledonous genus Rosa. Up to that time all polyploids.were regarded as simple duplications of chromosome sets,and no dis-tinction was made between duplicational polyploidy and differentialpolyploidy. The distinction between these two kinds of polyploidyis vital and fundamental. Perhaps the best way of expressing thedifference between them is that duplicational polyploids are polyploidvarieties, while differential polyploids are polyploid species.

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THE MECHANISM OF HEREDITY AND EVOLUTION. 27

Polyploid varieties have duplicated sets of chromosomes, whilepolyploid species have differential sets of chromosomes. The somaticformula of a tetraploid variety, where each letter represents a gametic setof chromosomes, is AAAA, while the formula of a tetraploid species isAABB, and so on with hexaploid, octoploid, and decaploid varietiesand species. Thus an octoploid variety is AAAAAAAA, while anoctoploid species is AABBCCDD.

In Rosa both polyploid varieties and polyploid species are found,but so far all the wild polyploids are polyploid species, and all thepolyploid varieties have apparently arisen under cultivation.

The genus Rosa is one of the largest and probably the most poly-morphic genus of plants and animals. The chromosomes of this genushave been more extensively studied perhaps than any other genus.Tackholm has counted the chromosomes of 293 species and forms,Blackburn and Harrison 30, Penland 9, and the writer 624, makinga total counted to date of 956 species and forms, representing all thevarious sections of the genus. Of these 369 are diploids with 7 gameticchromosomes, while 587 are polyploids with gametic chromosomes inmultiples of 7. The diploids have 14 somatic chromosomes, triploids21, tetraploids 28, pentaploids 35, hexaploids 42, and octoploids 56.

It will be observed that all the chromosome numbers in Rosa arein multiples of 7, and in the wild state, at all events, these septuplenumbers are maintained without exception. In consequence of this.multiple series of 7 in Rosa, the chromosome sets are known as septets.Unlike many other genera in plants and animals the chromosomes ofRosa are approximately equal in size, and no pronounced differences inshape have yet been recognised.

A detailed examination and tabulation of the characters of the 369diploid Roses combined with genetical experiments in crossing variousforms, demonstrate that in Rosa proper there are 5 differential septetsof chromosomes known as A, B, C, D and E septets. Each septet ofchromosomes carries the genes representing at least 50 differentialcharacters. Some of these characters are homozygous and specific whileothers are heterozygous and varietal. Some characters are morphologi -cal and taxonomic while others are physiological and ecological.

Consequently there are 5 diploid species in Rosa with septetformulae AA, BB, CC, DD and EE, where each letter represents aseptet of chromosomes carrying the linked genes of the specific char-acters.

A detailed examination and tabulation of the characters of the 587polyploids, combined with genetical experiments in crossing variousforms, demonstrate that 10 of these are polyploid varieties of severalspecies. These are simply varieties with duplicated septets of chromo-somes as in Oenothera, Datura and other plants, and in Ascaris andArtemia in animals. The remaining 577 polyploid Roses are quitedifferent in their nature, being polyploid species with differentialseptets of chromosomes instead of duplicated septets as in the poly-ploid varieties. Intensive analyses of these polyploid species andgenetical experiments show clearly that they are composed of variouscombinations of the A, B, C, D and E septets of chromosomes andcharacters of the 5 diploid species. On this basis 31 regular septet

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species of Rosa are possible in the genus. So far 23 of these have beenidentified, leaving 8 to be found (if they exist). As a matter of factone of the missing species has already been made genetically. Spacewill not allow one to deal with the numerous irregular septet speciespossible in these combinations, though a large number have alreadybeen worked out.

The chief point is that in the septet species of Rosa we arrive at anew conception of the nature of a species, and find a new mechanism forthe origin and evolution of species. As one species differs from anotherby one or more septets ofchromosomes and characters, septet species inthis genus are precise taxonomic units, and being subject to experi-mental verification by both genetical and cytological methods these-species are no longer abstract concepts as of old,but concrete realitiesand quantitative and qualitative entities.

Incidentally the septet species provides a new and precise methodof classification of the species of the genus.

Confirmation of the nature of differential polyploidy came in 1926in the Monocotyledonous genus Triticum.

The genus Triticum contains a large number of sub-species andvarieties of Wheats, which fall naturally into 3 groups of good species.The Einkorn group (T. monococcum),with 14 somatic and 7 gameticchromosomes. The Emmer group (T. dicoccum),with 28 somatic and14 gametic chromosomes. The Bread Wheats (T. vulgare), with 42-somatic and 21 gametic chromosomes. The chromosome sets inTriticum, as in Rosa, are in sevens or spetets. So far only 4 septets ofchromosomes and characters have been identified, A, B, C and D, assuggested by Gaines and Aase in 1926. T. monococcum is probably AA,T. dicoccum AABB,andT. vulgareAABBCC. It seems not unlikelythatthe closely related Aegilops cylindrica is CCDD, and Aegilops ovataBBDD. All the species except T. vulgare are from wild species, and-there is little doubt that Percival's suggestion of 1921 that the BreadWheats have evolved under cultivation by the hybridisation of theabove wild species is substantially correct.

Other genera in plants and animals have not yet been sufficientlyworked out to test the nature of their polyploidy, but many are nowworking at these problems and the future is full of promise. In themeantime among many others the Composite genera Chrysanthemumand Senecio, though not yet fully worked out in their chromosomes andcharacters, seem likely to have polyploid species like Rosa and Triti-.cum.

Tahara found multiple sets of chromosomes in 12 species ofChrysanthemum, the diploids had 9 pairs of chromosomes, the tetra-ploid 18 pairs, the hexaploid 27 pairs, the octoploid86 pairs, and thedecaploid 45 pairs of chromosomes.

The sets in Chrysanthemum are therefore in nines or novets insteadof septets as in Rosa and Triticum. All the polyploids in Chrysan-themum appear to belong to wild species, while the species usuallycultivated in gardens seem to be mostly diploid. In view of thecircumpolar distribution of the octoploid species of Rosa, it may besignificant that the decaploid species of Chrysanthemum is apparently-arctic.

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In the genus Senecio (Groundsel) Afzelius found in 40 species amultiple series of somatic chromosome numbers 10-20-40-50--(60-180,and in 20 species of 10 genera in the same Tribe (Senecioneae) hefound a similar series 1-20-40-- 50-60.

The lowest gametic number was 5, so that in Senecio and Sene-cioneae the chromosome sets are in fives or quintets instead of septetsas in Rosa and Triticum. In the different sections and sub-sections ofSenecio it is remarkable how constant the chromosome numbers areand the correlation between chromosomes and characters is clearlyclose.

It is evident that a wide field of investigation lies open in manylarge genera of plants and animals to determine the facts of differentialpolyploidy and its significance in heredity and evolution. In order toachieve this purpose intensive studies of the chromosomes and charac-ters of all the species and sub-species of the large and polymorphicgenera and families are necessary, and combined with these studies,genetical experiments are indispensable. An individual worker can'hardly expect in his life-time to complete the work in one large genuseven, but every genus thoroughly worked out taxonomically, cyto-logically and genetically will represent a forward step towards acomplete knowledge of heredity and evolution.

In the meantime it may be permissible to speculate on the possi-bilities and probabilities of the mechanism of evolution, but only sofar as our present experimental knowledge will take us.

Evolution of Rosa.In view of the presence of the characters and chromosomes of 2

or more diploid species in the polyploid species of Rosa, it is temptingto assume that the polyploid species have arisen by hybridisation ofthe 5 diploid species followed by a duplication ofthe chromosomes, andthat in the septet species we have a working mechanism for the originof species by hybridisation. The possibility of this has already beendemonstrated in Primula, Triticum, Narcissus and Nicotiana. Thatsome of the irregular septet species have arisen in this way is mostprobable, and it is not impossible that some of the regular septet speciesalso originated in this manner. There are however serious difficultiesin the way of accepting the theory of hybridisation as a completeexplanation of the origin of the genus as a whole. The facts of geo-graphical and ecological distribution seem to demand an alternativeexplanation.

These facts point rather to an origin by descent from an Arcticdecaploid species with 5 septets of chromosomes and characters, general-ised and adapted to extremely variable conditions of life, which bysuccessive losses of whole septets of chromosomes gave rise to the lowerpolyploid species, and eventually to the 5 specialised diploid species.The septet species mechanism of evolution however works either way,an ascent from diploid to polyploid species by hybridisation in a centri,-petal direction, or a descent from polyploid to diploid species by septetspeciation in a centrifugal direction. Most probably the mechanismalternates in both directions.

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Distribution of Species of Rosa.All the species of Rosa are confined to the northern Hemisphere.

The higher polyploid species, the octoploids with 56 chromosomes art-circumpolar in their distribution, while the diploid species with 14chromosomes extend south to the Tropics. The hexaploid species with42 chromosomes and the tetraploid species with 28 chromosomesoccupy more or less a middle position. This distribution coincideswith the view of the Arctic origin of the land flora of the-northern hemisphere, and points to an origin by descent from an arcticpolyploid through the loss of whole septets of chromosomes andcharacters. Loss of septets by disuse under extreme changes wouldbe natural, as each septet represents an ecological value expressed in-the ecological isolation of the 5 diploid species. In this sense extremechanges in the conditions of life would directly determine the trend of-speciation and provide the material for the natural selection of indivi -duals.

The octoploid species R. acicularis with 56 somatic chromosomeshas the septet formula BBCCDDEE. The crude ecological valuesof these septets (as expressed in the present distribution of the diploidspecies) are B=warm and dry, C=cold and dry, D=cold and wet,E=warm and wet. So that this species should be able to thrive underextremely variable conditions, and its distribution is circumpolar.I have received seeds of this species collected in Alaska, where itgrows 30 miles north of the Arctic Circle, and also from RussianLapland, where last summer Professor Gates found it growing evenfurther North of the Arctic Circle.

With regard to the evidence for septet speciation by loss of septets,in my material I have seen 7 cases of the actual loss of whole septetsof chromosomes in 2 polyploid species and 1 polyploid variety. It isremarkable too that these cases have only been observed in speciesthat have been subjected to extreme changes in their conditions oflife, and after centuries of cultivation in distant countries. One of thespecies is Rosa damascena, the actual plant of which is said to havebeen introduced from Persia to Italy and Spain about 1531.

The septet formula of this tetraploid species is AACC, and theexistence since 1800 of a triploid form growing with it in cultivation,-and said to be a bud sport, with the formula AAC, may be significant,since it suggests that in this case it is the cold and dry septet C thathas been lost in a temperate and wet climate. The second species is atriploid CDD, which arose from the tetraploid species CCDD R. virgin-iana introduced into England from Virginia about 1640.

This evidence shows that the loss of a septet of chromosomesgiving rise to a new septet species is a possible mechanism in evolu--tion, and also that it seems to be associated with extreme changes in theconditions of life maintained for a considerable period of time.

In view of the extreme changes that have undoubtedly occurredin secular time especially during the Ice Ages of the Pleistocene, onewould expect many such changes to occur.

The original decaploid species, representing on this view the genusRosa, would most probably arise by the duplication of the septets

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T!HE MECHANISM OF HEREDITY AND EVOLUTION. 31

-of an ancient diploid species under luxuriant conditions, possiblyafter a sudden chill, just as duplicated varieties have arisen and arearising to-day under cultivation and in a wild state. This duplicationof septets would be followed in secular time by a differentiation ofthe septets by duplicational segregations and gene mutations, thusgiving rise within itself to the potentiality to throw off numerousnew septet species in a later epoch according to the geologicalconditions.

In this way evolution would be an alternating process carried outby the mechanism of the chromosomes. In the primary phase therewould be first a quantitative increase of septets of chromosomes fromdiploid to polyploid, and then a qualitative change of genes in theseptets, which would be a truly creative phase in the sense that the new;septets of chromosomes with the new gene complexes representing thenew species would be formed, though as yet only partly expressed in thegeneralised polyploid species.

In the secondary phase the successive throwing off of the septetsof chromosomes would lead to a truly emergent phase in the sense thatthe new species would appear with increasing expression, until finallythe diploid species would emerge in which full and complete expressionwould be given to all the wide range of characters laid down in thecreative stages ages before, and now at last emergent in terms of eco-logical values.

In both of these phases it is evident that the mainspring of themechanism is to be found in the response of the chromosome sets of theorganism to the extreme changes that have taken place in the conditionsof life in different geological periods, so that the evolution of newspecies has been determined by the interaction between the organismand the conditions of life. These rhythmic cycles of alternatingcreative and emergent evolution, working by means of the mechanismof the chromosome sets in association with other processes such as genemutations, syngamy and hybridisation, duplication, non-disjunction,fusion and segmentation of the chromosomes, would also serve toexplain the origin of tribes, families, orders, classes, phyla and largerdivisions ofthe animal and vegetable kingdoms, including the numerousextinct species, genera and families eliminated by natural selection.The emergent species of to-day may be represented as the expression ofthe creative phases of past ages, and this creative process is evidentlycontinuing to-day to be expressed in the emergent species of the future.