76 Life and Death, Heredity and Evolution 



(M and N) consistently produce small shells. We also get 

 stocks hereditarily different for numbers of spines ; and for 

 numbers of teeth. Our single stock, derived by fission from 

 a single parent, has gradually diversified itself into many 

 stocks that are hereditarily different. If this is what we 

 mean by evolution, we have here seen evolution occur. 



In Figure 24 we see a number of the hereditarily diverse 

 stocks that arose by fission from a single parent, in one of 

 my experiments. It will be worth while to summarize the 

 main facts as to the appearance of hereditary variations in 

 this animal. 



(1) Hereditary variations arose in some few cases by 

 rather large steps, which might be called mutations, or 

 saltations. 



(2) But the immense majority of the hereditary varia- 

 tions were minute gradations. Variations are as continu- 

 ous as can be detected. 



(3) Hereditary variations occurred in many different 

 ways, on many diverse characters : the number of spines ; 

 the length of the spines ; the size of the body ; the num- 

 ber of teeth. There was no single line of variation that was 

 followed exclusively, or by the great majority of cases. 



(4) Any set of characters gave rise to variations in-* 

 dependently of the other characters. Thus many diverse 

 combinations of characters arose; large animals with long 

 spines ; small animals with long spines ; large animals with 

 short spines ; short animals with short spines, and so on 

 for other sorts of combinations. 



(5) The hereditary variations which arose were of just 

 such a nature as to produce from a single strain the heredi- 

 tarily different strains that are found in nature. 



I judge that if the intermediate strains were killed off, the 

 two most diverse strains found in nature might well be 



