398 EVOLUTION AND GENETICS 



organs. These organs are obviously in a state of gradual reduction 

 following the termination of their usefulness to the individual. 

 They are usually indifferent, but too uniform in occurrence to be 

 explained as indifferent structures incidentally retained. The 

 human appendix, for example, might readily be influenced by 

 natural selection to a point of indifference, since we see that it 

 is now a harmful structure through its tendency to become infected. 

 Beyond that point, however, its condition would be of no vital 

 importance, would neither eliminate nor preserve the individual, 

 and so could be changed only through the operation of other 

 forces. 



Overspecialization. Overspecialization may conceivably bring a 

 structure to a stage of development which destroys its usefulness 

 without rendering it actually harmful. The tusks of the Columbian 

 elephant, now extinct, became very large, and in some specimens 

 curved so extremely that their tips must have crossed (Fig. 207). 

 Such organs could not be used for digging or for defense in the way 

 that the tusks of existing elephants are used, yet there is nothing 

 to show that they were sufficiently burdensome to this magnificent 

 creature to account for its disappearance. 



The theory of natural selection is therefore widely applicable, 

 but is not a sufficient explanation of all evolution of species. 

 Darwin, in his earlier works, looked upon it as the chief process of 

 evolution, and some of his followers have given it even greater 

 emphasis, but even Darwin came to reaUze that other views were 

 not without a claim to serious consideration. The contributions 

 of the twentieth century have added definite theories no less 

 logical than Darwin's, if of less widespread appHcation, and to 

 these we may now turn our attention. 



Darwin's theory may be looked upon as one of several partial 

 explanations of evolutionary change. It is limited in that it does 

 not account for the origin of characters, and only incidentally 

 affects indifferent characters. It may readily be seen, however, 

 that the useful and harmful characters of indi\'iduals play an 

 important part in their lives, and that natural selection logically 

 accounts for the preservation and elimination of such characters 

 as components of the species. Within these limitations it may 

 be applied extensively to problems of evolution, Ixit beyond them 

 it must give way to other theories. It is more a theory of adapta- 

 tion than of species-formation. 



