between Man and all other Animals, 137 



exactly resemble. Human agency apart, I do not think there 

 is a single species which even approximates to universal distri- 

 bution. Of course, we can only judge from probability and 

 analogy. Great locomotive power, even the maximum, by 

 no means necessarily implies an extensive distribution : wit- 

 ness the common swift, and its American analogue*, neither 

 of which have been known to straggle across the Atlantic, 

 like many birds of far less power of wing, but are bound by 

 the localising principle. It is true, this principle can apply 

 only to such species as are locomotive ; but it is equally true, 

 that other causes analogously restrain the undue diffusion of 

 those which are individually fixed. Thus we hear of the 

 agency of sea currents in transporting seeds, which must 

 abundantly be carried out into the ocean by the action of 

 rivers ; but it appears not to have been remembered that 

 steeping in sea water destroys the vital principle ; that 

 moisture induces germination, which, once excited, can only 

 be checked by the final cessation of the vital functions, f 

 Analogy would lead us to infer that such antagonist prin- 

 ciples obtain throughout creation, whether or not human 

 observation may have yet detected their existence. It would 

 be easy to point out additional hindrances to the more exten- 

 sive spread of species of fixed habit, by treating on the frac- 

 tion which are allowed to attain maturity, even in their normal 

 habitat, of the multitude of germs which are annually pro- 

 duced ; and in what ratio the causes which prevent the 

 numerical increase of a species in its indigenous locality 

 would act where its adaptations are not in strict accordance 

 will sufficiently appear, on considering the exquisite perfec- 

 tion of those of the races with which it would have to con- 

 tend. If there is a probability that any species has become 

 naturally of general distribution, it is in the case of two 

 lepidopterous insects, Acherontia A'tropos and Cynthia car- 

 dui, both of which are of peculiarly erratic habits; and it is 

 said that these are found throughout the world : yet the dif- 

 ferences which exist in specimens from diverse localities are 

 hard to reconcile with specific variation, at least to judge 

 from what tropical specimens I have seen of the former ; 

 and an eminent entomological friend has remarked to me, in 

 conversation, that he is equally sceptical, judging from his 



* Chaetiira pelasgica. 



+ This observation is, however, intended to apply merely to those of 

 inland plants; for some maritime species, as the Pandaneae and cocoa 

 palms, have their seeds encased in sea-proof coverings, especially adapted 

 for floating uninjured on the waves : the restricted distribution of such 

 vegetables is provided for on another principle. 



