182 THE DESCENT OF MAN. [Part I. 



adaptations to similar habits of life : two animals may, 

 for instance, have had their whole frames modified for 

 living in the water, and yet they will not be brought any 

 nearer to each other in the natural system. Hence we can 

 see how it is that resemblances in unimportant structures, 

 in useless and rudimentary organs, and in parts not as yet 

 fully developed or functionally active, are by far the most 

 serviceable for classification ; for they can hardly be due 

 to adaptations within a late period ; and thus they reveal 

 the old lines of descent or of true affinity. 



We can further see why a great amount of modifica- 

 tion in some one character ought not to lead us to sepa- 

 rate widely any two organisms. A part which already 

 differs much from the same part in other allied forms has 

 already, according to the theory of evolution, varied 

 much ; consequently it would (as long as the organism 

 remained exposed to the same exciting conditions) be 

 liable to further variations of the same kind ; and these, 

 if beneficial, would be preserved, and thus continually 

 augmented. In many cases the continued development 

 of a part, for instance, of the beak of a bird, or the teeth 

 of a mammal, would not be advantageous to the species 

 for gaining its food, or for any other object ; but with man 

 we can see no definite limit, as far as advantage is con- 

 cerned, to the continued development of the brain and 

 mental faculties. Therefore in determining the position 

 of man in the natural or genealogical system, the extreme 

 development of his brain ought not to outweigh a multi- 

 tude of resemblances in other less important or quite un- 

 important points. 



The greater number of naturalists who have taken 

 into consideration the whole structure of man, including 

 bis mental faculties, have followed Blumenbach and Cu- 

 vier, and have placed man in a separate Order, under the 

 title of the Bimana, and therefore on an equality with the 



