ANIMAL EVOLUTION 31 



individual to selected individual, any amount of modi- 

 fication is possible, including abbreviation of the ontogeny, 

 the suppression of the larval stages, the precocious 

 segregation of the primary constituents ; everything, in 

 short, that is of advantage. I must confess that I am not 

 quite content with this explanation, and there are others 

 who are not content. But I have not the time to criticize 

 it now. To do so requires a full consideration of the 

 supplementary theories of Histonal Selection, Germinal 

 Selection, and Amphimixis, theories not to be lightly 

 treated of or explained in a phrase. 



Nor can I offer any explanation of my own of the 

 unquestionable fact that the modification of the germ 

 and the modification of adult structure have proceeded 

 pari passu, in the course of phyletic evolution. After 

 struggling for some time with the difficulty, I have 

 come to the conclusion that we must wait for fresh light 

 from the experimental schools of zoology. 



For the present, it is enough that we have ample 

 evidence of the pre-existence of ' primary constituents ' 

 or * factors ' in the germ. 



What the precise nature of these factors may be we 

 are hardly in a position to say at present. But a great 

 deal of light has been thrown on the behaviour of these 

 factors by the researches of the Mendelian School of 

 Students of Heredity, at Cambridge. It has been proved 

 that they are resident in the germ-cells, and that they 

 are units, in the sense that they are amenable to the 

 laws of number, and their actions can be represented 

 by simple arithmetical calculations. It has been clearly 

 proved, also, that in respect of any given structure or 

 character in an adult organism, the result manifested is 

 not, in many cases at least, due to the action of a single 

 factor, but to the co-operation of two or more factors. 

 For it has been shown that, for instance, the colour of 

 a sweet-pea is not determined by a single colour-bearing 

 factor, but by the co-operation of two factors. For if 



