6 GEOGKAPHICAL DISTRIBUTION. 



camel, gazelle, and ostrich, would present to him certain features 

 of a fauna which was in the main unknown to him ; in India the ele- 

 phant, lion, and rhinoceros, and other curious denizens of the jun- 

 gle, the python and crocodile, and the numerous birds of resplendent 

 plumage, would probably crowd from his memory the forms of the 

 creatures ordinarily most familiar to him, and lead a passage to the 

 ultimate goal of his journey, Australia, where he would meet with 

 the most singular and most distinctive fauna on the surface of the 

 earth. 



Much nearer to his northern home — on ojiposite sides of the 

 Mediterranean — and with much less travelling, the naturalist will 

 discern scarcely less well-marked faunal differences or peculiarities. 

 To account for the anomalies which the facts of distribution present 

 is the still unsolved problem that is put before the zoogeographer. 



Granting, with the doctrine of evolution, that all the complex 

 assemblages of existing animal forms are modified derivatives from 

 previously existing forms, and that these are ultimately to be traced 

 back to some common ancestor, it must of necessity follow that any 

 given fauna will depend for the degree of its peculiarity, whether 

 great or small, upon the amount of modification, relative to any 

 other fauna, which it will have undergone. And this modification 

 can be effected in two ways : by inherent modification of the indi- 

 vidual types composing the fauna, and by intermixture with, or 

 immigration from, contiguous or neighbouring faunas. In both 

 cases, manifestly, isolation or its opposite, union of habitation, will 

 constitute the governing factor in determining the amount of varia- 

 tion. A region that is broadly separated from all others will, natu- 

 rally, tend to develop a fauna distinct from any other, since the 

 progressive modifications in its constituent faunal elements must ul- 

 timately lead to divergence ; and the greater the period of isolation 

 the greater, of necessity, will be the amount of this divergence, or 

 the more pronounced the faunal individualisation. Hence it is that 

 in the greater number of the more distantly removed island groups, 

 or in those wiiich are separated by more or less impassable barriers 

 from the nearest land-mass, we meet with such highly specialised 

 faunas. The Galapagos Islands, for example, as will be more fully 

 illustrated farther on, have a fauna very distinct from that of any 

 part of South America, although removed from it by a distance of less 

 than seven hundred miles. The birds are quite distinct, and so are 



