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 
continually augmented. In many 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- 
