II 



Heredity and Variation in Protozoa, in Reproduction from 

 a Single Parent. "The Inheritance of Acquired Characters." 

 The Existence of Many Diverse Stocks in a Single Species. 

 Constancy of the Stocks. "Pure Line Inheritance" and the 

 Results of Selection. Difficulties for the Theory of Evolu<- 

 tion. 



first chapter led us to the question of heredity in 

 the Protozoa. In the present chapter we take up 

 the study of heredity and variation in the simplest kind of 

 reproduction, where the offspring are produced by the divi- 

 sion of a single individual into two. By way of introduction, 

 let us bring this into relation with the problem of life and 

 death, which we have already considered. We have seen 

 that these organisms are so constituted that they live in- 

 definitely, with no intervention of natural death of the in- 

 dividuals ; yet we have seen also that they continue to 

 reproduce. The inevitable result is that more individuals 

 are produced in each species than nature can provide space 

 and opportunity for, so that most of them are condemned 

 to violent and unnatural death. Is there anything changed 

 by this continual over-production, with destruction of the 

 majority? Are those produced exactly like those that 

 existed before? Or do the animals change as generations 

 pass, so that some are better fitted for the conditions that 

 they meet, and therefore continue to exist, while others are 

 killed off? That is, can we see evolution occur as we watch 

 these creatures through generation after generation? 



38 



