" The factors that refuse to combine and so produce inter- 

 mediate forms are those that form the subjects of Mendelian 

 research. 



" How do they acquire this antagonistic position to each 

 other ? The answer must be in the same way that species 

 acquire immiscibility, viz. by natural selection acting on 

 variation during periods of segregation. 



" Thus the factor for height in peas becomes in one race a 

 factor for tallness, in another a factor for shortness, which 

 prove to be immiscible. On the other hand, the factor for 

 human skin colour, that one would expect to be more immis- 

 cible than that for height in peas, proves to be (invariably?) 

 quite miscible. 



" To account for such differences, we may appropriate any 

 available explanations from the cases of possible species being 

 more or less fertile inter se. In the case of Mendelian factors 

 we may suppose that the selection has gone on long enough to 

 give the two forms of a factor specific rank as against each 

 other, just as it does in the case of any plant or animal. Where 

 miscibility exists we may suppose the necessary selection has 

 not continued long enough to secure fixity. We must j^robably 

 call in another element about which, however, little is 

 known. The two factors may be chemically (or otherwise) 

 incajDable of combining together. One would suppose this 

 capable of arising more easily where the new variety arose by 

 mutation, more difficult to picture as a result of gradual 

 change by selection. 



" The broad view would be that each species has a germ- 

 plasm consisting of an enormous number of factors, each of 

 which can act on its own part and without similar action on 

 the part of other factors, as if it were a species liable to variation 

 and selection. When the plasm of any two races (of plant or 

 animal) presents a sufficient number of differentiated factors, 

 or, more probably, when some important factors are sufficiently 

 differentiated, the two races become distinct species. 



" One may regard Mendel as having been especially fortunate 

 in selecting for experiment the garden pea, which lias an 

 unusual number of easily recognised differentiated factors. 

 It might be supposed that the garden pea has not been in 



