682 



SCIENCE. 



[N. S. Vol. XXIII. No. 592. 



especially to call your attention to the great 

 number of instances of the utility of struc- 

 tures and adaptation to environment ad- 

 duced by him. There is a mass of evidence 

 showing that organisms possess useful 

 structures and are more or less in harmony 

 with their environment. I, therefore, as- 

 sume both the theory of evolution and that 

 organisms are more or less perfectly adapt- 

 ed to their environment. 



There are three hypotheses to account 

 for the evolution of new species and their 

 perpetuation. Each one of the hypotheses 

 recognizes the struggle for existence and 

 the influence of natural selection. 



The first hypothesis is considered pe- 

 culiarly the Darwinian, to which Weis- 

 mann has added his theoretical supplement. 

 According to this hypothesis, animals and 

 plants are subject to fluctuating variations 

 of small amount. Those individuals whose 

 variation gives them an advantage over 

 their competitors for life are preserved, 

 producing a gradual amelioration. This 

 process is continued until ultimately the 

 descendants differ so widely from their 

 progenitors that they are referred to a 

 separate species. The Weismannian sup- 

 plement denied the hereditary transmissi- 

 bility of characters impressed on the indi- 

 vidual by its environment, so that the ad- 

 justment of organisms to their environ- 

 ment would depend entirely upon the 

 preservation of favorable fortuitous varia- 

 tions. 



The second hypothesis, usually known as 

 the neo-Lamarckian, called the dynamic by 

 Dall, and that of direct causation by 

 Nageli, for many years the rival of the one 

 just stated, undertakes to account for the 

 phenomena of adaptation by assuming that 

 the organisms are directly molded into 

 harmony with their environment by ex- 

 ternal forces, or that adaptation may orig- 

 inate through conscious effort, and that 



changes in individuals so brought about 

 are transmitted to their offspring. The 

 principal defenders of this hypothesis in 

 this country were Cope and Hyatt. Dall, 

 although he proposed the term dynamic, 

 I believe, never accepted Cope's hypothesis 

 of archsesthetism, an idea first advanced 

 by Lamarck. 



The third hypothesis is that of de Vries. 

 He admits the struggle for existence, and 

 recognizes fluctuating variation. He, how- 

 ever, contends that this kind of variation 

 will not give rise to a new species, basing 

 this conclusion on a large amount of ex- 

 perimental data obtained from the breed- 

 ing of plants. According to him all that 

 can be done by selecting the best indi- 

 viduals is to ameliorate the race to a cer- 

 tain point, beyond which no progress can 

 be made, and that so soon as the process 

 of continued selection is abated, regression 

 toward the average variation of the species 

 or variety ensues. De Vries also denies the 

 permanency of the effect of natural selec- 

 tion on the fluctuating variations of a spe- 

 cies. Natural selection, he claims, can not 

 accumulate variations of this type beyond 

 a certain limit; and that if individuals of 

 such a naturally ameliorated race be trans- 

 ferred to another area they will regress 

 toward the mean characters of the species. 

 Natural selection has preserved certain in- 

 dividuals of the species, but has not really 

 changed it. From a study of the mode of 

 the appearance of the permanent 'varieties' 

 or 'species' of cultivated plants, de Vries 

 was led to believe that new species do not 

 originate either by the gradual elimination 

 of unfavorable fluctuating variation or by 

 the direct modifying influence of environ- 

 ment, but by sudden mutation — the off- 

 spring differing from their parents by dis- 

 tinct lacunse, breeding true and showing no 

 tendency to revert to the parental form. 

 His acquaintance with cultivated plants 



