EVIDENCES FROM GEOGRAPHIC DISTRIBUTION 161 



enable us to give a rational account not only of the peculiarities of 

 form and structure presented by animals and plants, but also of their 

 grouping together in certain areas, and their general distribution over 

 the earth's surface. 



"In the absence of any exact knowledge of the facts of distribution, 

 a student of the theory of evolution might naturally anticipate that all 

 groups of allied organisms would be found in the same region, and that, 

 as he travelled farther and farther from any given centre, the forms 

 of life would differ more and more from those which prevailed at the 

 starting-point, till, in the remotest regions to which he could penetrate, 

 he would find an entirely new assemblage of animals and plants, 

 altogether unlike those with which he was familiar. He would also 

 anticipate that diversities of climate would always be associated with a 

 corresponding diversity in the forms of life. 



"Now these anticipations are to a considerable extent justified. 

 Remoteness on the earth's surface is usually an indication of diversity 

 in the fauna and flora, while strongly contrasted climates are always 

 accompanied by a considerable contrast in the forms of life. But 

 this correspondence is by no means exact or proportionate, and the 

 converse propositions are often quite untrue. Countries which are 

 near to each other often differ radically in their animal and vegetable 

 productions; while similarity of climate, together with moderate 

 geographical proximity, are often accompanied by marked diversi- 

 ties in the prevailing forms of life. Again, while many groups of 

 animals — genera, families, and sometimes even orders — are confined 

 to limited regions, most of the families, many genera, and even 

 some species are found in every part of the earth. An enumeration 

 of a few of these anomalies will better illustrate the nature of the 

 problem we have to solve. 



"As examples of extreme diversity, notwithstanding geographical 

 proximity, we may adduce Madagascar and Africa, whose animal and 

 vegetable productions are far less alike than are those of Great Britain 

 and Japan at the remotest extremities of the great northern continent; 

 while an equal, or perhaps even a still greater, diversity exists between 

 Australia and New Zealand. On the other hand, Northern Africa 

 and South Europe, though separated by the Mediterranean Sea, have 

 faunas and floras which do not differ from each other more than do 

 the various countries of Europe. As a proof that similarity of climate 

 and general adaptability have had but a small part in determining the 

 forms of life in each country, we have the fact of the enormous increase 



