132 THE 'ORIGIN OF SPECIES.' [i860. 



tempted by its novelty to make it too strong) ; yet it seems 

 to me, not really very killing, though I cannot answer your 

 case, especially, why Rodents have not become highly devel- 

 oped in Australia. You must assume that they have inhab- 

 ited Australia fcr a very long period, and this may or may 

 not be the case. But I feel that our ignorance is so pro- 

 found, why one form is preserved with nearly the same struct- 

 ure, or advances in organisation or even retrogrades, or be- 

 comes extinct, that I cannot put very great weight on the 

 difficulty. Then, as you say often in your letter, we know 

 not how many geological ages it may have taken to make any 

 great advance in organisation. Remember monkeys in the 

 Eocene formations : but I admit that you have made out an 

 excellent objection and difficulty, and I can give only unsat- 

 isfactory and quite vague answers, such as you have yourself 

 put ; however, you hardly put weight enough on the abso- 

 lute necessity of variations first arising in the right direction, 

 videlicet, of seals beginning to feed on the shore. 



I entirely agree with what you say about only one species 

 of many becoming modified. I remember this struck me 

 much when tabulating the varieties of plants, and I have a 

 discussion somewhere on this point. It is absolutely implied 

 in my ideas of classification and divergence that only one or 

 two species, of even large genera, give birth to new species ; 



and many whole genera become wholly extinct Please 



see p. 341 of the ' Origin.' But I cannot remember that I 

 have stated in the ' Origin ' the fact of only very few species 

 in each genus varying. You have put the view much better 

 in your letter. Instead of saying, as I often have, that very 

 few species vary at the same time, I ought to have said, that 

 very few species of a genus ever vary so as to become modi- 

 fied ; for this is the fundamental explanation of classification, 

 and is shown in my engraved diagram. . . . 



I quite agree with you on the strange and inexplicable 

 fact of Ornithorhynchus having been preserved, and Austral- 

 ian Trigonia, or the Silurian Lingula. I always repeat to 

 myself that we hardly know why any one single species is 



