1804.] 685 



jtt least a thousand of them ; and even then the occurrence of very 

 many identical species in faunas and floras which, as Greoloiry teaches 

 us. were separated by insurmountable physical barriers long before the 

 present geological epoch commenced, and have continued to be so sepa- 

 rated ever since, forms an almost insuperable objection to the hypothe- 

 sis. The only other assumption that we can make — after rejecting the 

 above two — is that species were not created in their present specific 

 types, but are genetically derived from pre-existing species. The 

 Tnity of Coloration, both as regards the shade of color and the 

 pattern or design, which prevails almost everywhere in Nature in the 

 same group of species, likewise indicates by unmistakeable tokens 

 a genetic connection between the different species of those groups. 

 There is actually, as I have attempted to show, a very general Phyto- 

 PHAGic Unity in those genera of insects which in the larva state feed 

 upon plants ; for it is very commonly the case that certain genera of 

 insects inhabit, more or less exclusively, certain genera of plants; and 

 I believe that when one species of a given genus of gall-making insects 

 is found on a given genus of plants, there can be almost universally 

 many more species of the same genus found there. At all events, the 

 (Jail-gnats of the Willow ofter a memorable illustration of this rule; 

 for before this Paper appeared but a single N. A. species was known 

 to the scientific world, and I have discovered at least fourteen additional 

 .species, and doubtless many more remain to be discovered. To say, b)'^ 

 way of explanation of these and similar phenomena, that they are so 

 because the Great Author of Nature has willed them to be so, is no 

 explanation at all, but simply a woman's reason — it is so, because it is 

 so. If I were to go into a large stable of horses, and find some of them 

 fed exclusively on maize, some exclusively on oats, some exclusively on 

 hay, and some, as usual, on an intermixture of the three kinds of feed. 

 I should naturally ask the horse-keepers what was the reason of this 

 singular anomaly. Would it be any answer for them to say — " It is 

 so, because the Master has willed it to be so"? What I should want 

 to know would be, lohy he willed it to be so, and what possible reason 

 he could have for such a proceeding ; and unless they could explain 

 this point, they might just as well hold their tongues. Now the Deri- 

 vative Theory explains full}' and completely what I called just now the 

 PnYTOi'HAGic Unity of numerous large groups of insects, and it also 



