C. B. Davenport 21 



organs and of more congenial habitats. Adaptation of organization to environment 

 has been effected by the double process of selection by environment of the most appro- 

 priate organization and by the organism of the most congenial environment. 



This hypothesis, I think, should be welcomed by those palaeontologists who, like 

 Osborn (1897), are led from their phylogenetic studies to conclude " that there are 

 fundamental predispositions to vary in certain directions." They help out, too, I 

 believe, the fundamentally important observations and experiment of DeVries (1901), 

 who finds that race change is a series of steps, of mutations, that may often have no 

 relation to adaptation ; the adaptation comes later. For all those theories, in general, 

 that assume that change of specific structure occurs independently of selection of the 

 fittest, the hypothesis here proposed must be considered a welcome complement. It 

 may be well to point out that the selection of a fitting environment is not confined to 

 migratory animals. It is applicable to all organisms that have a means of dispersal. 

 The seed that falls upon good ground — the race that gets into a favorable environ- 

 ment — will survive. 



The theory may thus be summarized: The world contains numberless kinds of 

 habitats, or environmental complexes, capable of supporting organisms. The number 

 of kinds of organisms is very great ; each lives in a habitat consonant with its struc- 

 ture. Each species is being widely dispersed, and, by chance, some members of a spe- 

 cies get into an environment worse fitted for them; others into one better fitted. 

 Those that get into the worse environment cannot compete with the species already 

 present ; those that get into a habitat that completely accords with their organization 

 will probably thrive and may make room for themselves, even as the English sparrow 

 has made room for itself in this country. This process may go on until the species is 

 found only in the environment or environments suited to its organization. As Dar- 

 winism is called the theory of the survival of the fittest organisms, so this may be 

 called the theory of segregation in the fittest environment. 



In conclusion I repeat that the theory of segregation in the fittest environment 

 does not replace that of survival of the fittest organism, but is complementary to it. 

 It has this raison d'etre that it shows how unadaptive mutations may become adaptive 

 if only they can find their proper place in nature. 



LITERATURE CITED 



Absolon, P. C. K. "Einige Bemerkungen tiber mahrische Hohlenfauna." Zool. Anz., Bd. 



XXIII (1900), pp. 1-6. 

 Balch, F. N. " List of Marine Mollusca of Coldspring Harbor, Long Island, with Descriptions 



of One New Genus and Two New Species of Nudibranchs." Proc. Boston Soc. Nat. 



Hist., VoL XXIX (1899), pp. 133-62. 

 Cooke, A. H. Mollusca in " Cambridge Natural History," Vol. Ill (1895), pp. 459. 

 De Vries, H. Die Mutationstheorie. Versuche und Beobachtungen uber die Entstehungen 



von Arten im Pflanzenreich. Leipzig: Veit & Co., 1901. 



175 



