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NATURE 



[October io, 1912 



cerned directly with the resokition of c into its com- 

 ponents. Indirectly it is concerned also with x; for 

 by the pursuit of the method the full value of c may 

 be determined, and hence that of x may be inferred. 

 This concession made, it is permissible to concentrate 

 our attention on the term c. 



Thus the business of the Mendelian is to resolve 

 the complex of characters which is possessed by an 

 individual into its constituent unit characters. As a 

 consequence of this experimental analysis Mendelism 

 is enabled to restate the problem of the behaviour 

 in inheritance of two individuals in these terms : — 



The complex of characteristics which distinguishes 

 an individual is the expression of the sum of a long 

 series of characters. As the individual arises from 

 germ cells so each character arises from a germ within 

 the germ cells. Such germs of characters are called 

 factors. When two germ cells unite to form an 

 incipient individual or zygote they bring together the 

 similar factors of a given character — one factor from 

 the one germ cell and the other from the other. As 

 the zygote forms the mature individual, so the paired 

 factors give rise to a character of the individual. 



The bodv characters are the flowers of the factorial 

 seeds imnlanted in the germ cells. 



Some characters are simple and derive from one 

 pair of factors only ; others are of an ascending 

 order of comnlexitv and may be traced to the co- 

 operative agency of two, or more than two, pairs of 

 factors. In the case of a complex character the deter- 

 mining factors may be unlike one another or they 

 may be alike. Thus two pairs of different factors 

 are required to produce the character of colour in 

 certain flowers; on the other hand, it is at least 

 probable that certain characters are the outcome of 

 repeated doses of the same factorial stimulant. Fur- 

 ther, the individual is a dual thing — a double-barrelled 

 gun. Each barrel is loaded with the factorial charge 

 supplied bv one of the two gametes by the union of 

 which its duality is constituted. Conversely and conse- 

 quently a gamete or germ cell is, in comparison with 

 the individual, of single and not of dual nature. It 

 has one barrel only, and therefore can carry or give 

 effect to one, and only one, of the two factorial 

 charges with which the individual was suDolied at the 

 time of its formation. 



Our image of the double-barrelled gun serves also 

 to illustrate the several states in which an individual 

 mav find itself with respect to its charge of factors 

 of anv given simple body character. 



Both barrels of the gun may be loaded. An indi- 

 vidual in like state possesses two factorial charges and 

 produces gametes, all of which are alike in the 

 possession of one of these factors. Therefore, such an 

 individual, when self-fertilised, or mated with its like, 

 produces gametes which are all alike in this respect, 

 and these eametes, fusing in pairs, give rise to indi- 

 viduals which all possess the character in question. 

 Such individuals are homozygous, they breed true to 

 the character. 



Neither barrel may be loaded; and an individual in 

 like state is also homozygous. It breeds true to the 

 absence of the character. If a gamete of the former 

 individual meet with one from the latter individual, 

 the resulting zygote is in like case with that of a 

 double-barrelled gun of which one chamber onlv is 

 loaded. The zygote is heterozygous for the character. 

 I'nlike the homozygotes, which breed true, the hetero- 

 z.vcrous individual does not breed true to the character 

 in question. 



Bv the aoolication of the foregoine propositions and 

 a little arithmetic, it mav be predicted that the off- 

 soring of the heterozvgote fall into three groups — 

 one homozygous for the character, and another hetero- 



NO. 2241, VOL. 90] 



zygous, and a third homozygous for the absence of 

 the character^ — and that, further, these types of indi- 

 viduals occur in the proportion of 1:2:1. Needless 

 to say, the prediction is susceptible of verification by 

 experimental breeding from the heterozygote. These 

 are Mendelian commonplaces with which I should 

 have hesitated to occupy our time were it not for the 

 fact that I desire to emphasise the epoch-making 

 nature of Mendel's -method. The magic wrought by 

 genius is potent because it is simple. The rules of 

 Mendelian method are simple. If it be urged that I 

 have broken my promise and strayed from method to 

 doctrine I would ask which of the simple propositions 

 I have stated may be demurred to bv any student of 

 biology ? 



The supreme importance of Mendel's contribution to 

 science consists in this : that instead of mixing any- 

 thing with anything "in the gruel thick and slab" 

 of a witches' cauldron, he has taught us to cast the 

 horoscope of Fate by the method of genetical analysis 

 of individual characters. Thus the first part of the 

 Mendelian restatement of the old problem of heredity 

 reads : Investigate one by one the modes of inheritance 

 of the several characters of an individual. Choose for 

 this purpose organisms which are as far as possible 

 alike in all respects except for the character under 

 investigation. Carry the experiment to its conclu- 

 sion, even to the third or fourth generation. If un- 

 certain results are obtained, ascertain before discard- 

 ing the method whether the uncertainty may not be 

 due to the interference of other characters not to be 

 suspected a priori of exercising an influence upon the 

 expression of the character under investigation. 



Who, for example, would suspect a morphological 

 character like thickness of stem of exercising an in- 

 fluence on the time of flowering of a plant? Yet such 

 is the case with the pea {Pisum sativum), and there 

 is evidence that when this disturbing influence is 

 removed inheritance of time of flowering follows 

 Mendelian rules. 



The second part of the restatement of the problem 

 of genetics may be expressed as follows : Only by the 

 use of individuals of proved constitution with respect 

 to a given character may the effect of external con- 

 ditions on organisms be determined. The study of 

 variation must be preceded by Mendelian analysis 

 and synthesis. Let me illustrate this theme by an 

 example. 



The species. Primula sinensis, the Chinese primrose, 

 has given rise to many distinct varieties. Among 

 these varieties are some with white flowers and others 

 with magenta, blue, red, or other coloured flowers. 

 Each of these varieties may be obtained of florists 

 in a pure strain — that is to say, in a strain which 

 breeds true to flower-character. For our immediate 

 purpose we will group these varieties into white and 

 coloured forms. 



It has been shown, however, that this apparently 

 natural mode of grouping is inadequate to give a 

 correct idea of the genetic constitutions of these races. 

 It would seem self-evident that the white races differ 

 from the coloured races by the lack of flower-pigment ; 

 vet Mendelian analysis demonstrates that there are 

 more subtle differences between the different races. 

 These differences become apparent when true-breeding 

 white and coloured plants are crossed with one 

 another ; for it is then discovered that two types of 

 white-flowered plants exist, and it is only by their fruits 

 — their offspring — that ye may know them. Thus if 

 certain white-flowered races are chosen for the experi- 

 ment, the result of crossing white and colour is a 

 coloured F, generation. If certain other white races 

 are used and mated with the coloured form the off- 

 spring of the cross all bear white flowers. The 



