328 Geographical Distribution of Animals 



view of the permanence of the oceans he accounts for the colonisation 

 of outlying islands by further elaborating the views of Lyell and 

 Darwin, especially in his fascinating Island Life, with remarkable 

 chapters on the Ice Age, Climate and Time and other fundamental 

 factors. His method of arriving at the degree of relationship of the 

 faunas of the various regions is eminently statistical. Long lists of 

 genera determine by their numbers the affinity and hence the source 

 of colonisation. In order to make sure of his material he performed 

 the laborious task of evolving a new classification of the host of 

 Passerine birds. This statistical method has been followed by many 

 authors, who, relying more upon quantity than quality, have obscured 

 the fact that the key to the present distribution lies in the past 

 changes of the earth's surface. However, with Wallace begins the 

 modern study of the geographical distribution of animals and the 

 sudden interest taken in this subject by an ever widening circle of 

 enthusiasts far beyond the professional brotherhood. 



A considerable literature has since grown up, almost bewildering 

 in its range, diversity of aims and style of procedure. It is a chaos, 

 with many paths leading into the maze, but as yet very few take us 

 to a position commanding a view of the whole intricate terrain with 

 its impenetrable tangle and pitfalls. 



One line of research, not initiated but greatly influenced by 

 Wallace's works, became so prominent as to almost constitute a 

 period which may be characterised as that of the search by specialists 

 for either the justification or the amending of his regions. As class 

 after class of animals was brought up to reveal the secret of the true 

 regions, some authors saw in their different results nothing but the 

 faultiness of previously established regions ; others looked upon 

 eventual agreements as their final corroboratiou, especially when for 

 instance such diverse groups as mammals and scorpions could, with 

 some ingenuity, be made to harmonise. But the obvious result of 

 all these efforts was the growing knowledge that almost every class 

 seemed to follow principles of its own. The regions tallied neither in 

 extent nor in numbers, although most of them gravitated more and 

 more towards three centres, namely Australia, South America and 

 the rest of the world. Still zoologists persisted in the search, and the 

 various modes and capabilities of dispersal of the respective groups 

 were thought sufficient explanation of the divergent results in trying 

 to bring the mapping of the world under one scheme. 



Contemporary literature is full of devices for the mechanical 

 dispersal of animals. Marine currents, warm and cold, were favoured 

 all the more since they showed the probable original homes of the 

 creatures in question. If these could not stand sea-water, they 

 floated upon logs or icebergs, or they were blown across by storms ; 



