Janxtaby 15, 1915] 



SCIENCE 



97 



But quantitative differences such as distin- 

 guish species are precisely those which do not 

 Mendelize in crosses. Bateson says (p. 291) : 

 Of the descent of quantitative characters we 

 still know practically nothing. 



By which he probably means that we know 

 nothing Mendelian, since in this address he 

 treats Mendelism as the all-sufficient basis of 

 evolution, and ignores a decade of intensive 

 work in America directed toward the discovery 

 of Mendelian factors as a basis for quantita- 

 tive differences, a work participated in by 

 many different workers, all favorably dis- 

 posed toward the idea, but all unavailing. For 

 the uniform result of a cross which involves 

 quantitative differences is the production of 

 an intermediate, which in turn produces inter- 

 mediates only slightly more variable than the 

 races originally crossed. Dimorphism or 

 sharply discontinuous polymorphism is reg- 

 ularly wanting after size crosses. This is a 

 fatal objection to the idea that specific differ- 

 ences are diseontinuovis in origin. One who 

 advocates this idea has no choice at present 

 but to ignore (as Bateson does) all evidence 

 derived from the experimental study of this 

 subject. 



The idea that large differences can not arise 

 by summation of small ones is rendered im- 

 probable by this evidence. For if the larger 

 (quantitative) differences can so readily be 

 broken down into smaller ones, it seems highly 

 probable that the process is reversible. In- 

 deed the experience of breeders shows that 

 it is. The dog-breeder alluded to by Bateson 

 who titrated his colored fluids to illustrate 

 blood dilution in crosses was, so far as quan- 

 titative characters are concerned, employing 

 a very apt method, notwithstanding Bateson's 

 disapproval of it. 



Even sport variations, which truly Men- 

 delize, and which form the basis of color vari- 

 eties and other fancy varieties among animals 

 and plants, even these are capable of second- 

 ary break-down or " fractionation," as Bateson 

 admits. In making this admission he differs 

 from the supporters of the pure-line theory 

 who conceive that a Mendelian factor is in- 

 capable of change, but who apparently hold 



the idea as an article of faith rather than one 

 requiring proof. 



Secondary break-down or modification of 

 Mendelian factors is, however, coming to be 

 so generally recognized that a special name is 

 now applied to its products, that of multiple 

 allelomorphs. Even those who hold to the con- 

 ception of " pure lines " now recognize that 

 the same sport variation (" mutation " or 

 " locus ") may assume several different condi- 

 tions which viewed quantitatively form a 

 graded series; but they insist on the discon- 

 tinuity of the grades or forms which a Men- 

 delizing character may assume, maintaining 

 on a priori grounds that these stages can not 

 be bridged. The perilousness of such a posi- 

 tion is apparent from a single well-known case. 

 The first discovered Mendelizing character in 

 animals was albinism and it is one of the 

 simplest and clearest cut of all Mendelian 

 characters thus far discovered. It was not to 

 be expected that the single step between a 

 wholly uncolored and the normally colored 

 condition would be or could be bridged. Yet 

 two such intermediate stages have already 

 been demonstrated, which are unmistakable 

 allelomorphs of albinism, i. e., which behave 

 as alternative forms of the same genetic factor. 

 If two such intermediate stages may arise, 

 why may not others arise; why not a dozen, 

 why not a thousand ? Is it safe to assume that 

 this is not possible? 



Bateson urges that in cases of color variation 

 such as that of the sweet-pea and the primrose 

 the large changes came first and the smaller 

 ones later by secondary break-down or " frac- 

 tionation." The argument implies, indeed, he 

 expressly claims, that large differences can 

 not be built up from small ones. I do not 

 believe that either paleontology or the history 

 of breeding will support Bateson in this claim. 

 On the former ground Osborn' holds to a 

 gradual origin of discontinuous differences 

 between organisms. A study of breeds of 

 animals in comparison with their wild orig- 

 inals or present-day representatives shows 



' Osborn, H. F., ' ' The Continuous Origin of 

 Certain Unit-characters as Observed by a Paleon- 

 toogist," Amer. Nat., 46, pp. 185-278, 1912. 



