Chap. XVin. CHANGED CONDITIONS. 131 



to the size, vigour, and fertility of their ofi'spring ; so, on tho 

 other hand, certain other changes in the conditions of life 

 cause sterility ; and as this likewise ensues from crossing 

 inuch-modified forms (;r species, we have a parallel and double 

 series of facts, which apparently stand in close relation to 

 each other. 



It is notorious that many animals, though perfectly tamed, 

 refuse to breed in captivity. Isidore Geoffrey St.-Hilaire^ 

 consequently has drawn a broad distinction between tamed 

 animals which will not breed under captivity, and truly 

 domesticated animals which breed freely — generally more 

 freely, as shown in the sixteenth chapter, than in a state of 

 nature. It is possible and generally easy to tame most 

 animals ; but experience has shown that it is difficult to get 

 them to breed regularly, or even at all. I shall discuss this 

 subject in detail ; but will give only those cases which seem 

 most illustrative. My materials are derived from notices 

 scattered through various works, and especially from a Report, 

 kindly drawn up for me by the officers of the Zoological 

 Society of London, which has especial value, as it records 

 all the cases, during nine years from 1838-46, in which the 

 animals were seen to couple but produced no offspring, as well 

 as the cases in which they never, as far as known, coupled. 

 This MS. Report I have corrected by the annual Rej^orts 

 subsequently published up to the year 1865.^^ Many facts 

 are given on the breeding of the animals in that magnificent 

 work, ' Gleanings from the Menageries of Knowsley Hall,' by 

 Dr. Gray. I made, also, particular inquiries from the expe- 

 rienced keeper of the birds in the old Surrey Zoological 

 Gardens. I should premise that a slight change in the treat- 

 ment of animals sometimes makes a great difference in their 

 fertility ; and it is probable that the results observed in 



* ' Essais de Zoologie Generale,' kept, and of these 1 species in 1*9 



1841, p. 256. have bred at least once during the 



^" Since the appearance of the first 20 years ; of 28 Marsupialia, 1 in 2*5 



edition of this work, Mr. Sclater has have bred ; of 74 Carnivora, 1 in 3*0 



published (' Proc. Zoolog. Soc.,' 1808, have bred; of 52 Rodentia, 1 in 4*7 



p. 623) a list of the species of mam- have bred; and of Quadrumana 75 



mals which have bred in the gardens species have been kept, and 1 in 6*2 



from 1848 to 1867 inclusive. Of the have bred. 

 Artiodactyla 85 species have been 



