Correspondence on Biology, etc. 



catastrophes by wMch the union and disunion with the 

 nearest mainland may readily be accomplished. . . . 

 — Believe me ever most truly yours, Cha. Lyell. 



Sir C. Lyell to A. R. Wallace 



73 Harley Street. May 2, 1867. 



My dear Sir, — I forgot to ask you last night about an 

 ornithological point which I have been discussing with the 

 Duke of Argyll. In Chapter V. of his ''Reign of Law" 

 (which I should be happy to lend you, if you have time to 

 look at it immediately) he treats of humming-birds, saying 

 that Gould has made out about 400 species, every one of 

 them very distinct from the other, and only one instance, 

 in Ecuador, of a species which varies in its tail-feathers in 

 such a way as to make it doubtful whether it ought to rank 

 as a species, an opinion to which Gould inclines, or only as 

 a variety or incipient species, as the Duke thinks. For the 

 Duke is willing to go so far towards the transmutation 

 theory as to allow that different humming-birds may have 

 had a common ancestral stock, provided it be admitted that 

 a new and marked variety appears at once with the full 

 distinctness of sex so remarkable in that genus. 



According to his notion, the new male variety and the 

 female must both appear at once, and this new race or 

 species must be regarded as an " extraordinary birth." My 

 reason for troubling you is merely to learn, since you have 

 studied the birds of South America, and I hope collected 

 some humming-birds, whether Gould is right in saying that 

 there are so many hundred very distinct species without 

 instances of marked varieties and transitional forms. If this 

 be the case, would it not present us with an exception to the 

 rule laid down by Darwin and Hooker that when a genus is 

 largely represented in a continuous tract of land the species 

 of that genus tend to vary ? 



23 



