424 ANNUAL, KEPORT SMITHSONIAN INSTITUTION, 1918. 



gories immensely reduce the part that natural selection may have to 

 play in the evolution of new species. With this reduced function it 

 becomes quite comprehensible. 



VII. 



The dominating tendency of modern research is thus to come to a 

 synthesis of the two chief factors of evolution — the Buffon-Lamarc- 

 kian factor, including the variations called forth by a changing envi- 

 ronment, and the Darwin- Wallacian factor of natural selection. Dar- 

 win, as we saw, frankly acknowledged it. 



Herbert Spencer had already come to this conclusion, only giving 

 even more importance to the first factor : 



The foregoing chapters, he wrote, in the second enlarged edition of his 

 principles of Biology, imply that neither extreme (i. e.. natural selection alone, 

 or the direct action of environment without the aid of natural selection) is here 

 adopted. Agreeing with Mr. Darwin that both factors have been operative, I 

 hold that the inheritance of functionally caused alterations has played a larger 

 part than he admitted even at the close of his life ; and that, coming more to the 

 front as evolution has advanced, it has played the chief part in producing the 

 highest types. 



It is most interesting to note that Weismann, although his starting 

 point was quite different from that of Darwin and Spencer, also came, 

 after all, to the same views. He began by proclaiming the " all- 

 sufficiency of natural selection " for giving origin to new species, and 

 rejected the necessity of inheritable adaptive changes being produced 

 by the environment. But we saw how he gradually came to new 

 hypotheses, which actually recognized the part played in the evolu- 

 tion of new species by inherited variation. 



Pages could be covered to show how biologists engaged in experi- 

 mental work came, after some hesitation, to recognize the modifying 

 influence of environment. But a few quotations will do to show the 

 general tendency of modern research. 



Standfuss has summed up the results of his 24 years' experiments 

 in a carefully worded lecture. He sees in the predominance of an 

 older type upon a newly appearing variation the key to the difficulty 

 of a transmission of acquired characters to the offspring. The grip 

 of the old stirp — of what has become strongly established during a 

 succession of generations — can not, Standfuss says, be easily over- 

 powered by the new (a view, by the way, expressed already by Bacon) . 

 And after having proved by his experiments that sometimes the new 

 is inherited, Standfuss concludes his lecture with these words : 



The mutual interaction between the agencies of the outer world and the 

 organisms gives origin to fluctuating (schwankenden) new forms; they are 



