358 eepoet— 1879. 



this peculiar race ' must be attributed to those gradual modifications produced by- 

 causes at present little understood, by which most of the striking variations met 

 with in the human species have been brought about — modifications more strongly 

 expressed the more completely isolated the race has become, and the farther 

 removed from its original centre of distribution.' I think, then, that though we 

 have not data for conclusively answering the question as to how far migration 

 (together with isolation) may be necessary for specific genesis, it is certain that it 

 is of very great efficacy and importance, and that credit is justly due to Buffon for 

 his early appreciation of its importance. 



The next question to which I woidd advert is that concerning ttte direct 



ACTION UPON ORGANISMS, OF THE EXTERNAL CONDITIONS WHICH SURROUND THEM. 



Buffon's belief was x that changes of specific form were brought about by change 

 of temperature and climatic change generally, as well as by change of food. 



The curious effects of stimulating food on colour — as of cayenne pepper with 

 canaries, and hemp-seed with parrots — are notorious. The direct action of the 

 environment on organisms has, I think, been of late somewhat undervalued. 

 Amongst evidences in favour of its importance, I would refer to some of Mr. Alfred 

 Wallace's observations. 2 He tells us that in the small island of Amboina, the 

 butterflies (twelve species, of nine different genera) are larger than those of any 

 of the more considerable islands about it, and that this is an effect plainly due to 

 some local influence. In Celebes, a whole series of butterflies are not only of a 

 larger size, but have the same peculiar form of wing. The Duke of York's island 

 seems, he tells us, to have a tendency to make birds and insects white or at least 

 pale, and the Philippines, to develop metallic colours, while the Moluccas and 

 New Guinea seem to favour blackness and redness in parrots and pigeons. Species 

 of butterflies which in India are provided with a tail to the wing, begin to lose that 

 appendage in the islands, and retain no trace of it on the borders of the Pacific. 

 The ^Eneas group of Papilios never have tails in the equatorial region of the 

 Amazon Valley, but gradually acquire tails, in many cases, as they range towards the 

 northern and southern tropics. Mr. Gould says that birds are more highly coloured 

 under a clear atmosphere than in islands or on coasts — a condition which also seems 

 to affect insects, while it is notorious that many shore plants have fleshy leaves. 

 I need but refer to the English oysters mentioned by Costa, which, when trans- 

 ported to the Mediterranean, grew rapidly like the true Mediterranean oyster, and 

 to the twenty different kinds of American trees, said by Meehan to differ in the 

 same, manner from their nearest European allies, as well as to the dogs, cats, and 

 rabbits which have been proved to undergo modifications directly induced by- 

 climatic change. 



It appeara then that much may be said in favour of that direct effect of sur- 

 rounding circumstances on Organisms in which Buffon believed. 



Lastly, I would refer to Buffon's belief that new species have arisen by degra- 

 dation. This again is an opinion which, after a period of disfavour, or at least of 

 neglect, has been of late revived, and has acquired considerable influence. I 

 may here refer to Anton Dohrn, who has recently advocated the very widely spread 

 and effective action of degradation as a cause of specific change. It will, I think, 

 be generally admitted that such exceptional Copepod crustaceans as Tracheliastes 

 and Lerneocera are due to degradation, and the probability seems to me very strong 

 that the Bhizocephala, at least many cirripeds, and the certoid worms, are also 

 degraded organisms. Very interesting would it be to know whether existing 

 Ascidians are also examples of degradation, as not a few zoologists now suppose ; 

 but most interesting of all is that parasite of cuttle fishes, Dicyema, the structure 

 of which has been recently investigated by Professor Edward Van Beneden, and 

 made the type of a new primary division of animals. Should this small worm-like 

 organism hereafter turn out to be a degraded form, it will show what an extreme 



development, but may have some other explanation not at present apparent.' I 

 would suggest that in this case — as in the large brains of Cetaceans — it may be due 

 to the need in their climate of generating much heat to sustain the necessary tem- 

 perature of the body. 



1 Op. cit. vol. xiv. p. 317. * See Tropical Nature, pp. 254-259. 



