412 CLASSIFICATION. Chap. XIII. 



species. Consequently the groups which are now large, 

 and which generally include many dominant species, 

 tend to go on increasing indefinitely in size. I further 

 attempted to show that from the varying descendants of 

 each species trying to occupy as many and as different 

 places as possible in the economy of nature, there is a 

 constant tendency in their characters to diverge. This 

 conclusion was supported by looking at the great diversity 

 of the forms of life which, in any small area, come into 

 the closest competition, and by looking to certain facts 

 in naturalisation. 



I attempted also to show that there is a constant 

 tendency in the forms which are increasing in number 

 and diverging in character, to supplant and exterminate 

 the less divergent, the less improved, and preceding 

 forms. I request the reader to turn to the diagram 

 illustrating the action, as formerly explained, of these 

 several principles; and he will see that the inevitable 

 result is that the modified descendants proceeding from 

 one progenitor become broken up into groups subordi- 

 nate to groups. In the diagram each letter on the 

 uppermost line may represent a genus including several 

 species ; and all the genera on this line form together 

 one class, for all have descended from one ancient 

 but unseen parent, and, consequently, have inherited 

 something in common. But the three genera on the 

 left hand have, on this same principle, much in common, 

 and form a sub-family, distinct from that including the 

 next two genera on the right hand, which diverged from 

 a common parent at the fifth stage of descent. These 

 five genera have also much, though less, in common ; 

 and they form a family distinct from that including 

 the three genera still further to the right hand, which 

 diverged at a still earlier period. And all these genera, 

 descended from (A), form an order distinct from the 



