Chap. VI. AFFINITIES AND GENEALOGY. 189 



words and points of construction, they would be uni- 

 versally recognised as having sprung from a common 

 source, notwithstanding that they differed greatly in 

 some few words or points of construction. But with 

 organic beings the points of resemblance must not con- 

 sist of 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 modifi- 

 cation in some one character ought not to lead us to 

 separate 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 con- 

 ditions) be liable to further variations of the same kind ; 

 and these, if beneficial, would be preserved, and thus 

 continuallv augmented. In manv cases the continued 

 development of a part, for instance, of the beak of a 

 bird, or of the teeth of a mammal, would not be advan- 

 tageous 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 concerned, to the continued de- 

 velopment 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 multitude of resem- 



