468 Proceedings of the Royal Physical Society. 



the dynamical impulse in evolution. As he had hinted in this 

 work, he subsequently laid great emphasis on the action of 

 sexual reproduction as a condition of variation ; and, in a 

 sense, the stimulus of the male element may be ranked 

 among the external influences. Very important in all 

 attempts to estimate the action of the environment is 

 AVeismann's recently developed position, that individual char- 

 acteristics impressed upon an organism, in response to outside 

 influences, cannot be shown to be hereditarily transmissible. 

 Only when the influence has had time to saturate through the 

 organism, and affect the reproductive elements, can the in- 

 heritance of acquired characteristics be allowed. But a due 

 discussion of this caveat will require a separate paper. 



Eoux, who has especially attacked the problem of the 

 mechanics of development, distinguishes self -differentiation, 

 where the specific nature of the modification is determined 

 by the energies of the system, from correlative differentiation, 

 or change determined by action and reaction between the 

 system and its environment. In marked antithesis to the 

 opinion that environment has a direct transforming action, 

 it is interesting to notice the statement of Meehan, who has 

 worked especially at the botanical side of the subject. He 

 says " environment seems to have no further influence than 

 to incite to action a change already ripe for development. 

 Variations are in accordance with a prior plan by which 

 nature itself is bound." 



In concluding this historical summary, which only refers 

 to some of the many important contributions to the theory 

 of environmental influence, and which it is proposed to 

 develop elsewhere, some notice of Spencer's last utterance 

 must be given. In his " Factors of Organic Evolution," he 

 maintains that "the direct action of the medium was the 

 primordial factor of organic evolution. Acting alone, it must 

 have initiated the primary differentiation in all units of 

 protoplasm alike. Variations in the surrounding influences 

 over an area would effect small contrasts in degree and kind 

 of differentiation. As soon as these became decided, natural 

 selection came into play. The differentiating action of the 

 medium never ceased to be a co-operator in development as 



