ioo PROBLEMS OF GENETICS 



weight it certainly has, but I cannot yet regard it as forming a 

 fatal objection to the application of factorial conceptions on 

 the grand scale. It may be recalled that we are no longer under 

 any difficulty in supposing that differences of all classes may be 

 caused by the presence or absence of factors. It seemed at first 

 for example that such characters as those of leaf shape might 

 be too subtle and complex to be reducible to a limited number of 

 factors. But first the work of Gregory on Primula Sinensis 

 showed that several very distinct types of leaves were related 

 to each other in the simplest way. In that particular example, 

 intermediates are so rare as to be negligible, but subsequently 

 Shull dealing with such a complicated example as Capsella, and 

 Leake in regard to Cottons, both forms in which intergrades occur 

 in abundance, have shown that a simple factorial scheme is 

 applicable. We need not therefore, to take an extreme case, 

 doubt that if it were possible to examine the various forms of 

 fruit seen in the Squashes by really comprehensive breeding 

 tests, even this excessive polymorphism in respect of structural 

 features would be similarly reducible to factorial order. 



It must always be remembered also that in a vast number 

 of cases, nearly allied forms which are distinct, occupy distinct 

 ground. Moreover, by whatever of the many available mechan- 

 isms that end be attained, it is clear that nature very often does 

 succeed in preventing intercrossing between distinct forms so 

 far that the occurrence of that phenomenon is a rarity under 

 natural conditions. The facts may, I think, fairly be summarized 

 in the statement that species are on the whole distinct and not 

 intergrading, and that the distinctions between them are usually 

 such as might be caused by the presence, absence, or inter-com- 

 bination of groups of Mendelian factors; but that they are so 

 caused the evidence is not yet sufficient to prove in more than a 

 very few instances. 



The alternative, be it explicitly stated, is not to return to the 

 view formerly so widely held, that the distinctions between 

 species have arisen by the accumulation of minute or insensible 

 differences. The further we proceed with our analyses the more 

 inadequate and untenable does that conception of evolutionary 



