28 ORGANIC EVOLUTION 



CHAPTER IV 



THE CONDITIONS OF ORGANIC EVOLUTION- 

 HEREDITY AND VARIATION 



The proposition that animal and vegetable species 

 change and yet that the changes attain some 

 measure of racial persistence, presupposes two 

 facts which I may call the conditions of evolution 

 — the facts known as heredity and variation. 



The fact of heredity, to which I have devoted 

 an entire volume of this series, is beset with 

 a thousand difficulties, and involves a host of 

 problems which interest the man of science not 

 only for themselves but for their practical im- 

 portance—to which I must allude in considering 

 the teaching of evolution as to human destiny. 

 But for our present purpose heredity is a simple 

 matter — none simpler. When people ask one, " Do 

 you believe in heredity ? " they use the term in 

 a sense which is as uncertain as it is unjustifiable. 

 There is no room for disbelief in the assertion that 

 a cat does not give birth to oaks or sheep but 

 always to kittens. This is- a fact of heredity, 

 which simply means that like tends to beget like ; 

 that men " do not gather tigs of thistles." 



Now it is self-evident that the fact of heredity, 

 if it were not balanced by any other, would be not 

 merely inimical to, but totally exclusive of, the pos- 

 sibility of evolution. If like always begat exactly 

 like, then the present descendants of the primal 

 organisms would be as they were. 



