II CAUSES OF PROGRESSIVE EVOLUTION 55 



many shoots are stunted and many are metamorphosed in 

 manifold ways, but others die away. I have on occasion 

 used another simile for evolution : 



" Thus we may," I said, " compare the whole process of the 

 modification of forms to the results of the migration of a 

 people over an extensive foreign territory. Some tribes, not 

 having the strength to follow, soon, others later, remain behind 

 (genepistasis), others again reach a distant goal. Some 

 retain their characters in their new home or strengthen them, 

 even modify them by correlation, others change under the 

 influence of external conditions and adapt themselves to the 

 environment all that is not sufficiently capable of endurance 

 is left lying by the way and perishes, and if the struggle for 

 existence is at all severe only the toughest of all survive. 

 The sooner the connection between the separate tribes is lost, 

 the sooner each appears as a new species, as a new genus ; 

 but all bear the stamp of common descent." 



I willingly confess, however, that this simile, like all 

 similes, is incomplete. 



I find, then, the ultimate and the most essential causes of 

 progressive evolution in all the causes of growth in general 

 therefore in all the influences of the external world upon 

 organisms. 



It has been said that it is impossible to comprehend how 

 the varying external conditions should produce a continual 

 advance in evolution (Weismann). In answer to which the 

 first proposition to be emphasised is, that each higher stage 

 of evolution attained is a firmly established condition, by the 

 previous argument, all the more firmly established, fastening 

 the more upon the new form, the longer it continues; a 

 condition which, therefore, the longer it exists is with the 

 greater difficulty disestablished, but which merely through 

 the continual repetition of stimuli, although these have no 

 unusual strength, will undergo a change in the course of 



