8 GEOGRAPHICAL DISTKIBUTIOK. 



bird that ever visits the islands, for, if the case were otherwise, it 

 would be incredible that no more common forms should have been 

 detected there. But the fact that the bobolink has remained abso- 

 lutely identical with the common form of South America, whence, 

 doubtless, most of the species of Galapagos birds have been derived, 

 while all the other birds of the island group have undergone more 

 or less modification since the islands were first tenanted, proves that 

 variation in its case has been prevented by the perpetuation of nor- 

 mal characters through interbreeding with the continental migrants. 

 In other w^ords, the breed has been kept true. Were the migrations 

 of the visitors checked or interrupted, there can be little question 

 that the island breed of bobolinks would undergo the same kind 

 of modification which distinguishes the other birds, and which has 

 developed in them new specific or varietal types. In the conti- 

 nent of Australia, again, we meet with the most remarkable exam- 

 ple of a highly specialised fauna being developed as the result of 

 long-continued isolation. Of all the varied mammalian forms which 

 elsewhere crowd the surface of the earth we have here but the 

 merest trace, for, with the exception of the rodents and bats, none 

 of the ordinary orders — Carnivora, Ungulata, Insectivora, &c. — are 

 represented.* And even of the rodents there is but a single family, 

 that of the mice (Muridse). On the other hand, the implacental 

 mammals — kangaroos, wombats, duck-bill — whose only non-Aus- 

 tralian representatives are the American family of opossums (Didel- 

 phidae), acquire here a wonderful development, and exhibit a diver- 

 sity of type-structure not met with in any other order of mammals. 

 Now, the animals of this class, or such as might be considered most 

 nearly allied to the marsupials, are the first of the Mammalia to ap- 

 pear in geological time, and they alone have thus far been detected 

 in any of the deposits (Triassic, Jurassic) of the middle geological 

 period, or Mesozoic era. They constitute the most primitively or- 

 ganised members of their class, and probably stand not far removed 

 from what may ultimately be proved to be the bottom of the mam- 

 malian series. 



In order to explain the anomalies of the Australian mammalian 

 fauna we must have recourse to the hypothesis of isolation, for in 



* The Australian wild-dog, or dingo, may prove to be indigenous, in which 

 case it would represent the Carnivora. 



