60 Life and Death, Heredity and Evolution 



itary constitution. On the contrary they are merely lasting 

 hereditary diversities between stocks whose origin has not 

 been observed. The situation is very greatly confused when 

 these diverse stocks continually intercross, as happens in so 

 many higher organisms, but it is not thereby essentially 

 changed. 



The situation in these lower organisms is very much what 

 it would be in man if in man new individuals were regularly 

 produced by the division of those already existing. We 

 have reason to believe that this practically does occur in the 

 case of identical twins ; they are produced by the division 

 of a single egg, which if it had not divided would have pro- 

 duced but one individual (see Newman, 1917). If this 

 occurred in man regularly and frequently, as it does in most 

 Protozoa, we should find that the human population con- 

 tained great numbers of individuals as precisely alike as are 

 identical twins. Each of us would meet his precise counter- 

 part at every turn. All these closely similar individuals of 

 one type, taken together, would correspond to a single one 

 of the stocks or races of the Protozoa. And as in the 

 Protozoa, there would exist great numbers of such diverse 

 stocks; in man as many as there -now exist hereditarily dif- 

 ferent individuals. That is, each person present in this 

 room would represent a diverse stock or race; for each 

 person has a constitution hereditarily diverse from every 

 other (save in the case of identical twins). 



Now this fact that a species consists of a great number 

 of hereditarily diverse stocks or races, often differing in 

 only minute particulars, throws a most unexpected, and to 

 many persons unwelcome, flood of light on many supposed 

 studies of evolution, and particularly on the effects of 

 selection in the breeding of organisms, giving to such 

 studies a significance quite diverse from that which they 





