ORIGIN OF THE SPECIFIC TYPE 317 



of the organization in Cetaceans and Birds as due to adaptation, we 

 must conclude that, in the rest of the great groups of the animal 

 kingdom, the main and essential parts of the structure are adaptations 

 to the conditions of life, even although the relations between external 

 circumstances and internal organization are not so readily recognizable. 

 For if there were an internal evolutionary force at all, we should 

 be able to recognize its operation in the origin of the races of Ceta- 

 ceans and Birds ; but if there be no such power, then even in cases 

 where the conditions of life are not so conspicuously divergent as in 

 Cetaceans and Birds, we must refer the typical structure of the group 

 to adaptation. Thus everything about organisms depends upon 

 adaptation, not only the main features of the organization, but the 

 little details in as far as they possess selection-value ; it is only what 

 lies below this level that is determined by internal factors alone, 

 by germinal selection ; but this is not an imperative force in the 

 sense in which the term is used by Nageli and his successors, for it is 

 capable of being guided ; it does not necessarily lead to an invariable 

 and predetermined goal, but it can be directed according to circum- 

 stances into many different paths. But it is precisely this that 

 constitutes the main problem of the evolution theory — how develop- 

 ment due to internal causes can, at the same time, bring about 

 adaptation to external circumstances. 



This lecture had been transcribed so far, and was ready for the 

 press, when I received the first volume of a new work by De Vries, 

 in which that distinguished botanist develops new vieM's in regard to 

 the transformation of species, based upon numerous experiments, 

 carried on for many years on the variation of plants. As not only 

 his views, but the interesting facts he sets forth, seem to contradict 

 the conclusions as to the transmutation of oi'ganisms which I have 

 been endeavouring to establish, I cannot refrain from saying some- 

 thing on the subject. 



De Vries does not believe that the transformation of species can 

 depend on the cumulative summation of minute ' individual ' varia- 

 tions ; he distinguishes between ' variations ' and ' mutations,' and 

 attributes only to the latter the power of changing the character 

 of a species. He regards the former as mere fluctuating deviations 

 which may be increased by artificial selection, and may even, with 

 difficulty, if carefully and purely bred for a long period, be made 

 use of to give character to a new breed, but which play no part at all 

 in the natural course of phylogeny. As regards phylogeny, he main- 

 tains that only 'mutations' have any influence, that is, the larger 

 or smaller saltatory variations which crop up suddenly and which 



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