10 THE MAMMALIA. 



is inexplicable. 1 Certainly all the phenomena here 

 referred to are intelligible when the supposition of 

 migration does not stumble upon contradictions 

 and surmountable obstacles, and when the capacity 

 of the organism to acclimatise itself using the 

 word in its widest sense is taken into account as 

 a long since established fact; but the question as 

 to the origin of species in general is left as a point 

 to be considered apart. Without doubt that which 

 tends to the widest distribution of an animal form 

 and to the intercourse between the most different 

 species, is the sea. Since recent scientific investi- 

 gations have made us as intimately acquainted 

 with the ocean-currents as with the systems of 

 rivers, with the range of cold currents and tongues 

 of water in the southern seas and conversely, and 

 has marked the different depths of the ocean 

 currents, and given us charts of the bottom of 

 the sea, with maps showing its elevations and 

 depressions it would seem that, with an account 

 of the animals in the sea, the possibilities and 

 causes of their occurrence would likewise be ex- 

 hausted. 



The state of the matter is very different -as 

 regards the distribution of animals on land, in 



1 Wallace, The Geographical Distribution of Animals. 



