PHYSICAL CONDITIONS IN GENESIS OF SPECIES. 897 



of the country at these localities; small size, in opposition to a general 

 law, occurring over northern noncalcareous districts, and larger size 

 more to the southward, where the underlying rock is limestone. In 

 this case the difference obviously results from the direct action of the 

 conditions of habitat upon every individual rather than from " slowl}^ 

 effected changes "' originating in " only a few of the inhabitants " of 

 these respective districts. 



Use and disuse of organs, through changes of habit resulting from 

 changed conditions of environment, must result in some modification 

 of the organ involved. As an example may doubtless be cited the 

 passerine birds of some of the smaller, remotely situated islands, as 

 the Guadeloupe and Galapagos groups, where recent investigations 

 liave shown that most of the species differ similarly in several features 

 from their nearest allies of the mainland, and of which they are 

 unquestionably insular forms. These differences consist in the 

 greater size of the bill, shorter wings, longer tails, and darker colors. 

 The sedentary life necessitated by the confined habitats of species 

 thus situated would naturally act more or less strongly on the organs 

 of flight, and a reduction in the size of the wing would folloAV, not 

 necessarily through the roundabout process of natural selection, 

 through the modification originally of a single individual, but by the 

 direct action on all the individuals alike of the changed conditions of 

 Hfe. 



It is doubtless unnecessary to further multiply examples of the 

 modification of animals by the direct action of the conditions of life. 

 The subject is one that can be but imperfectly treated at best in a 

 short paper like the present. The illustrations have here been drawn 

 from a limited geographical field and mainly from among the two 

 jiigher classes of vertebrates. There are, however, abundant indica- 

 tions that other fields and other classes would yield results equally 

 confirmatory of the direct action of physical conditions in the evolu- 

 tion of specifi'C forms among animals and plants. Changes in en- 

 vironing conditions will, however, go but a short way toward explain- 

 ing the origin of the great diversity of structure among existing 

 organisms; the character of the food, habit, or the increased use or the 

 disuse of particular organs may explain many of the modifications, 

 leaving a large share of the work to as yet unknown causes. Natural 

 selection, as sometimes defined, is made to cover all causes of dif- 

 ferentiation, it being stated by Mr. Darwin himself that if organic 

 l)eings undergo modification through changes in their conditions of 

 life " uniformity of character can be given to their modified offspring 

 solely by natural selection preserving similar favorable variations." 

 In its strict sense, variation by natural selection results only through 

 favorable differences appearing at first in single isolated individuals, 

 which transmit these favorable qualities to their offspring, in virtue 



