Characters as Adaptive and Specific. 165 



creasingly cogent the higher the taxonomic value 

 of the structure. Indeed, it may be laid down as 

 a general rule, that the greater the degree of adapta- 

 tion the greater is its diffusion — both as regards 

 the number of species which present it now, and 

 the number of extinct species through which it has 

 been handed down, in an ever ramifying extension 

 and in an ever improving form. Species, therefore, 

 may be likened to leaves : successive and transient 

 crops are necessary for the gradual building up of 

 adaptations, which, like the woody and permanent 

 branches, grow continuously in importance and 

 efficiency through all the tree of life. Now, in my 

 view, it is the great office of natural selection to see 

 to the growth of these permanent branches ; and 

 although natural selection has likewise had an enor- 

 mously large share in the origination of each suc- 

 cessive crop of leaves — nay, let it be granted to the 

 ultra-Darwinians for the sake of argument, an ex- 

 clusive prerogative in this respect — still, in my view, 

 this is really the least important part of its work. 

 Not as an explanation of those merely permanent 

 varieties which we call species, but as an explanation 

 of the adaptive machinery of organic nature, which 

 has led to the construction both of the animal and 

 vegetable kingdoms in all their divisions do I regard 

 the Darwinian theory as one of the greatest general- 

 izations in the history of science. 



I have dwelt thus at some length upon a mere 

 matter of definition because, as we shall now find, 

 although it is but a matter of definition, it is fraught 

 with consequences of no small importance to the 



