486 DE. ALFEED E. WALLACE ON" 



of the species, at all comparable witli the well-marked cLaractera 

 that usually distinguish, insular forms, and there is nothing in 

 mere isolation without selection which can increase the difference. 

 As examples we may refer to the many peculiar speciea of 

 butterflies and birds found in the various islands of the "West 

 Indian and Malayan Archipelagoes, which are quite as distinct 

 from each other as are allied continental species, and which 

 exhibit all the characteristics of forms which have been fully 

 differentiated by natural selection. 



The sketch now given of the usual mode of formation of new 

 species under natural selection leads to the conclusion that every 

 species (of the higher animals at all events) will usually possess 

 at least three peculiarities : in the first place, it must exhibit 

 some difference of structure or function adapting it to new con- 

 ditions ; secondly, some distinction of colour, form, or peculiar 

 ornament serving as distinctive recognition marks ; and, thirdly, 

 the physiological peculiarity of some amount of infertility when 

 crossed with allied species. The first two constitute its " specific 

 characters." But if we consider that every species in the long 

 line of its ancestry must have had similar specific characters, 

 adapting it to the peculiar conditions of its environment and 

 distinguishing it from its nearest allies ; that some of these 

 characters, when generally useful, have persisted, and now con- 

 stitute generic or family characters ; that others have been again 

 and again modified so as to adapt them to new and sometimes 

 quite different conditions ; and that others again, becoming use- 

 less, persist when quite harmless or remain in a more or less 

 rudimentary condition ; and when we further consider that many 

 genera and families extend far back into geological time and must 

 have originated in the midst of a physical and biological environ- 

 ment very different from that which now prevails, we shall dimly 

 understand how complex are the forces and processes which have 

 led to the assemblage of characters now presented by each 

 organism, and how difficult it must be to determine positively 

 that any one of these characters is not, nor ever has been, useful 

 to its possessor. Yet this is what is done by those writers who 

 maintain, as did the late Mr. Romanes, that the majority of 

 specific characters are not and never have been useful, but 

 have arisen through definite variation under the influence of 

 definite causes, and, when neither useful nor hurtful, persist and 

 constitute the main external differences which we observe between 



