THE MUTATION THEORY 99 



cases of specific diversity between near allies? Postponing the 

 problem of the interrelations of the larger divisions as altogether 

 beyond present comprehension, can we suppose, that in general, 

 closely allied species and varieties represent the various con- 

 sequences of the presence or absence of allelomorphic factors 

 in their several combinations? The difficulty in making a 

 positive answer lies in the fact that in most of the examples in 

 which it has been possible to institute breeding experiments with 

 a view to testing the question, a greater or less sterility is en- 

 countered. Where, however, no such sterility is met with, as 

 for instance in the crosses made by E. Baur among the species 

 of Antirrhinum there is every reason to think that the whole 

 mass of differences can and will eventually be expressed in terms 

 of ordinary Mendelian factors. Baur has for example crossed 

 species so unlike as Antirrhinum majus and molle, forms differing 

 from each other in almost every feature of organisation. 1 The 

 F z generation from this cross presents an amazingly motley 

 array of types which might easily if met with in nature be de- 

 scribed as many distinct species. Yet all are fertile and there 

 is not the slightest difficulty in believing that they can all be 

 reduced to terms of factorial a'nalysis. 



If allowance be made for the complicating effects of sterility, 

 is there anything which prevents us from supposing that such 

 good species as those of Veronica or of any other genus comprising 

 well-defined forms may not be similarly related? I do not know 

 any reason which can be pointed to as finally excluding such a 

 possibility. Nevertheless it has been urged with some plausi- 

 bility that good species are distinguished by groups of differ- 

 entiating characters, whereas if they were really related as the 

 terms of a Mendelian F 2 family are, we should expect to find 

 not groups of characters in association, but rather series of forms 

 corresponding to the presence and absence of the integral factors 

 composing the groups of characters. I am not well enough 

 versed in systematic work to be able to decide with confidence 

 how much weight should be attached to this consideration. Some 



1 See Lotsy and Baur, Rep. Genetics Conf., Paris, 1911, pp. 416-426. Com- 

 pare Lecoq on Mirabilis jalapa X longiflora, Fecondation des Vegetaux, 1862, p. 311. 



