308 



SCIENCE. 



[N. S. Vol. XXIII. No. 582. 



abled to seize on many and widely diversified 

 places in the polity of nature, and so be en- 

 abled to increase in numbers " (p. 87). " The 

 more diversified in habits and structure the 

 descendants of our carnivorous animals be- 

 come the more places they will be enabled to 

 occupy" (p. 88). "I will now give two or 

 three instances both o£ diversified and of 

 changed hahits in the individuals of the same 

 species. In either case it would be easy for 

 natural selection to adapt the structure of the 

 animal to its changed habits, or exclusively 

 to one of its changed habits. It is, however, 

 difficult to decide, and immaterial for us, 

 whether habits generally change first and 

 structure afterwards" (p. 141). In view of 

 the fact that in the ordinary cases the changes 

 of structure are not adaptive, it seems to me 

 quite material to recognize the change of 

 habits as in itself adaptive and as an impor- 

 tant condition of selection. " He who believes 

 in the struggle for existence and in the prin- 

 cipal of natural selection will acknowledge 

 that every organic being is constantly en- 

 deavoring to increase in numbers; and that 

 if any one being varies ever so little, in either 

 habits or structure, and thus gains any ad- 

 vantage over some other inhabitant of the 

 same country, it will seize on the place of that 

 inhabitant, however different it may be from 

 its own place. Hence it will cause him no 

 surprise that there should be geese and frigate 

 birds with webbed feet living on dry land and 

 rarely alighting on the water; that there 

 should be long-toed corncrakes living in mea- 

 dows instead of in swamps; that there should 

 be woodpeckers where hardly a tree grows; 

 that there should be diving thrushes and 

 diving Hymenoptera, and petrels with the 

 habits of hawks" (p. 145). These quotations 

 may give an idea of Darwinism quite different 

 from that suggested by some current defini- 

 tions which the authors have not felt obliged 

 to connect with Darwin's writings. 



By ecological adaptation is meant the kind 

 of adaptation which occurs when a species 

 occupies a favorable position without showing 

 any obvious adaptive characters, except such 

 as are common to other members of the same 

 genus or larger group. By ecological selec- 



tion is meant the kind of selection which is 

 conditioned upon the species occupying a 

 favorable position without developing any 

 obvious adaptive characters. 



No ecological position is favorable for an 

 unlimited number of individuals. The mul- 

 tiplication of species results from the fact 

 that the dominant species produce more indi- 

 viduals than can occupy the same position. 

 Whenever the number exceeds the optimum, 

 even when there is no kind of inferiority 

 among the individuals, wholesale extermina- 

 tion must occur, or some of the individuals 

 must avoid competition with the dominant 

 form by a change of place or habits. 



Change of place seems to be the easiest and 

 most natural means of avoiding competition 

 and one of the most obvious conditions of 

 selection. I am inclined to regard this as the 

 most important factor and the one which will 

 explain the most cases. I think the students 

 of geographical distribution can show a thou- 

 sand incipient species where the mutationists 

 can show a doubtful one. In a local fauna it 

 is remarkable how few species belong to the 

 same genus. It is almost certain that the 

 nearest relatives of any form will be found 

 outside the district. The homogeneous ele- 

 ments diverge from a given habitat and the 

 heterogeneous elements converge there. What 

 happens to the migrating form seems to me 

 of less importance, if it can be shown that the 

 migration was a condition of selection. This 

 may be hard to show. It is obvious enough, 

 if we contemplate the return of all of the 

 forms to their original starting point. Modi- 

 fication of the geographical segregate by the 

 intercrossing of its more or less isolated mem- 

 bers, by the operation of the selective condi- 

 tions of the new environment, or by the local 

 influences which give an impress to large ele- 

 ments of the fauna, are all secondary to the 

 selective conditions which induced the migra- 

 tion. 



But a considerable proportion of species 

 may have originated in the same place. In 

 this case the condition of selection is the 

 adoption of habits which relieve them from 

 the pressure of competition with the dominant 

 forms. If a form occupies a favorable posi- 



