Alfred Russel Wallace 



metaphor of the river' is new to me, and admirable ; but your 

 other metaphor, in which you compare classification and 

 complex machines, does not seem to me quite appropriate, 

 though I cannot point out what seems deficient. The point 

 which seems to me strong is that all naturalists admit that 

 there is a natural classification, and it is this w^hich descent 

 explains. I wish you had insisted a little more against the 

 North British'' reviewer assuming that each variation which 

 appears is a strongly marked one; though by implication 

 you have made this very plain. Nothing in your whole 

 article has struck me more than your view with respect to 

 the limit of fleetness in the racehorse and other such cases; 

 I shall try and quote you on this head in the proof of my 

 concluding chapter. I quite missed this explanation, though 

 in the case of wheat I hit upon something analogous. I am 

 glad you praise the Duke's book, for I wa« much struck with 

 it. The part about flight seemed to me at first very good, 

 but as the wing is articulated by a ball-and-socket joint, I 

 suspect the Duke would find it very difficult to give any 

 reason against the belief that the wing strikes the air more 

 or less obliquely. I have been very glad to see your article 

 and the drawing of the butterfly in Science Gossip. By 

 the way, I cannot but think that you push protection too 



^ See WaUace, Quarterly Journ. of Sci.y 1867, pp. 477-8. He imagined an 

 observer examining a great river system, and finding everywhere adaptations 

 wliich reveal the design of the Creator. " He would see special adaptations to 

 the wants of man in the broad, quiet, navigable rivers, through fertile alluvial 

 plains, that would support a large population, while the rocky streams and 

 mountain torrents were confined to those sterile regions suitable for a small 

 population of shepherds and herdsmen." 



* At p. 485 Wallace deals with Fleeming Jenkin's review in the North British 

 Review, 1867. The review strives to show that there are strict limitations to 

 variation, since the most rigorous and long-continued selection does not in- 

 definitely increase such a quality as the fleetness of a racehorse. On this Wallace 

 remarks that the argument " fails to meet the real question," which is not 

 whether indefinite change is possible, but " whether such differences as do occur 

 in nature could have been produced by the accumulation of variations by 

 selection." 



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