164 Heredity. 



reason to doubt whether life has existed long enough to 

 permit the evolution of existing forms in this way, and 

 natural selection gives no account of the sudden appear- 

 ance of considerable modifications, although the history 

 of domestic animals shows us that such saltations do 

 sometimes occur. 



On the one hand we find that Darwin's assumption 

 that variations are fortuitous involves us in grave difficul- 

 ties, but on the other hand we find scarcely any evidence 

 to show that permanent hereditary race modifications 

 are ever directly produced by the action of external con- 

 ditions, while we do find evidence for the opinion that 

 race modifications are, as a rule, not due to this direct 

 action, but to congenital variation. 



Our theory furnishes an explanation which lies mid- 

 way between Darwin's view of the origin of variation 

 and the Lamarkian view, and thus enables us to escape 

 both these difficulties, for it shows us how the influence 

 of changed conditions upon an organism may give rise 

 to congenital variation in later generations, and it also 

 shows us why variations tend to appear at the time and 

 place where they are needed. It also shows how a con- 

 siderable modification may appear suddenly and become 

 hereditary. 



The correlated variation of homologous organs and 

 the correlated modification of the various parts of a 

 complicated organ are accepted by Darwin without ex- 

 planation, but the theory of heredity shows us that these 

 phenomena, the chance against the fortuitous occurrence 

 of which is almost infinite, are due to the working of a 

 very simple law. 



When we review the ground and see how all the phe- 

 nomena of hybridization and variation fall into their 

 proper places; how the same simple explanation fits the 



