NATURAL SELECTION. 13 



view accords well with Maupertuis's philosophical axiom of 

 " least action." 



In considering how far the theory of natural selection 

 may he extended, — that is, in determining from how many 

 progenitors the inhabitants of the world have descended, — 

 we may conclude that at least all the members of the same 

 class have descended from a single ancestor. A number of 

 organic beings are included in the same class, because they 

 present, independently of their habits of life, the same funda- 

 mental type of structure, and because they graduate into each 

 other. Moreover, members of the same class can in most 

 cases be shown to be closely alike at an early embryonic age. 

 These facts can be explained on the belief of their descent from 

 a common form ; therefore it may be safely admitted that all 

 the members of the same class are descended from one pro- 

 genitor. But as the members of quite distinct classes have 

 something in common in structure and much in common in 

 constitution, analogy would lead us one step further, and to 

 infer as probable that all living creatures are descended from 

 a single prototype. 



I hope that the reader will pause before coming to any 

 final and hostile conclusion on the theory of natural selection. 

 The reader may consult my ' Origin of Species ' for a general 

 sketch of the whole subject ; but in that work he has to take 

 many statements on trust. In considering the theory of 

 natural selection, he will assuredly meet with weighty 

 difficulties, but these difficulties relate chiefly to subjects — 

 such as the degree of perfection of the geological record, the 

 means of distribution, the possibility of transitions in organs, 

 &c. — on which we are confessedly ignorant; nor do we know 

 how ignorant we are. If we are much more ignorant Than 

 is generally supposed, most of these difficulties wholly 

 disappear. Let the reader reflect on the difficulty of looking 

 at whole classes of facts from a new point of view. Let him 

 observe how slowly, but surely, the noble views of Lyell on 

 the gradual changes now in progress on the earth's surface 

 have been accepted as sufficient to account for all that we see 

 in its past history. The present action of natural selection 

 may seem more or less probable: but I believe in the truth of 



