RELATIVE VIABILITY IN MAMMALS AND BIULS. 543 



British birds are often very difficult to keep in captivity. I have 

 shown that this difficulty occurs in so lai-ge a number of differejit 

 kinds of British birds and mammals, that it may be laid down as 

 a cui'ious pi-inciple (to which naturally some exceptions exist) that 

 British birds and mammals have a lower viability in captivity in 

 England than their immediate allies from any other pai't of the 

 world. This remarkable circumstance is in the first place, in the 

 strict sense of the word, accidental, and finds partial explanation 

 in a cause independent of the constitution of the animals. Most 

 captured examples of wild species either are weakly individuals, 

 or are injured or frightened by the process of captvire : a heavy 

 mortality is to be expected. In the case of animals that come from 

 a distance, mvich of the heavy mortality takes place before ari-ival 

 or, because of the additional evil efi"ects of the conditions during 

 transit, and the survivors which reach their destination are rela- 

 tively strong and hardy. On the other hand, local animals 

 reach their destination in a shorter time, and the heavy mortality 

 takes place iuside the Gardens. But this explanation is iiot wholly 

 sufficient to cover the cases, and I think it may be assumed that 

 wild birds and mammals in Britain have acquired an intolerance 

 of man, without which, unfortunately, they would not have been 

 allowed to maintain their existence. This psychological acquisition 

 presses heavily on them in captivity. It appears to be the case 

 that an opposite pi^ocess of selection is taking place in the parks 

 of great cities, and that wild birds in particular are learning not 

 to fear man. It would be interesting to know if wild birds 

 taken in a London park lived better in captivity than birds of 

 the same species from country districts. 



(7) The climate from which a bird or mammal comes has the 

 smallest possible relation to its viability in captivity. A. Heilprin 

 [Disto^ihution of Animals, Int. Sci. Series, vol. Iviii., 1887, p. 3.5) 

 pointed out the error of the common belief as to climate being 

 the principal factor that regulates or controls the distribu- 

 tion of animals. Amongst mammals and birds a vast majority 

 of species and genera regarded as tropical have an actual or 

 recent range into tempeiate or even frigid cliu^ates. Of those 

 now limited to the ti-opics, still fewer are accustomed to a steady 

 temperature. Some range periodically or occasionally to altitudes 

 whei'e great cold occurs ; otheis, by exposure to the intense 

 radiation of the diy air of plains at night, regularly endure cold 

 going down to freezing-point; whilst many inhabitants of tropical 

 forests (which we naturally associate with steamy heat) must be 

 subjected to great cold in their nocturnal wanderings on the 

 summits of tall trees. The abundance of thick hair and fur and 

 of close feathering amongst tropical creatures is a clear indication 

 that their life is not spent basking in tropical sunlight. Mammals 

 a.nd birds have the power of maintaining their internal temperatuie 

 at a normal that varies only within an extremely narrow range, 

 notwithstanding the tempei'atui'e changes in their environment ; 

 and I do not doubt, not only that they can endure considerable 



