40 Darwin, and after Darwin. 



Thus, for instance, " nothing can be easier than to 

 define a number of characters common to all birds; 

 but with crustaceans any such definition has hitherto 

 been found impossible. There are crustaceans at the 

 opposite ends of the series, which have hardly a 

 character in common ; yet the species at both ends, 

 from being plainly allied to others, and these to others, 

 and so onwards, can be recognised as unequivocally 

 belonging to this, and to no other class of the arti- 

 culata '." Now it is evident that this progressive 

 modification of specific types — where it cannot be 

 said that the continuity of resemblance is anywhere 

 broken, and yet terminates in modification so great 

 that but for the connecting links no one could divine 

 a natural relationship between the extreme members 

 of the series, — it is evident that such chains of af- 

 finity speak most strongly in favour of a transmutation 

 of the species concerned, while it is impossible to 

 suggest any explanation of the fact in terms of the 

 rival theory. For if all the links of such a chain 

 were separately forged by as many acts of special 

 creation, we can see no reason why B should re- 

 semble A, C resemble B, and so on, but with ever 

 slight though accumulating difterences, until there is 

 no resemblance at all between A and Z. 



I hope enough has now been said to show that all 

 the general principles and particular facts appertaining 

 to the natural classification of plants and animals, are 

 precisely what they ought to be according to the 

 theory of genetic descent ; while no one of them is 

 such as might be — and, indeed, used to be — expected 



' Origin of species, pp. 368-9. 



