50 THE PROBLEM OP DISPERSAL 



brought about such permanent structural changes as con- 

 stitute distinct species, there are also instances of organic 

 constancy having prevailed to preserve the species un- 

 changed through vast periods of time, in regions widely 

 and impassably separated from each other. Separated, 

 that is, not only by thousands of miles in space, but by 

 the barrier of salt water, in which fish of these species 

 cannot exist for five minutes. 



Consider, for example, the suggestions offered by the 

 genus Oalaxias, consisting of about fourteen species of 

 small fresh-water fishes confined to the southern hemi- 

 sphere. In New Zealand they were dubbed trout by the 

 early settlers, and certainly in their spotted skins and 

 general outline they bear a superficial resemblance to our 

 own brook trout, but they have no scales, and are closely 

 related, not to the salmonidoe, but to the pike family. 

 They are, in fact, as Dr. Gunther puts it, the pikes of the 

 southern hemisphere. Of this genus six species have 

 been identified in Tasmania and south-eastern Australia, 

 five in New Zealand and the Auckland Islands, and four 

 in the Patagonian region of South America. Now there 

 would be nothing surprising in the occurrence of different 

 species of the same genus Qalaxias in different parts of 

 this enormous area, for there is a close general affinity 

 between the fresh-water fishes of Australasia and South 

 America, accounted for partly by similarity of physical 

 environment. But the important fact is that one species, 

 Oalaxias attenuatus, is identical in Tasmania, New 

 Zealand, the Falkland Islands, and the Fuegian region of 

 South America. Great is the significance enveloped in 

 this apparently insignificant creature of three or four 

 inches long. It almost amounts to an axiom of evolution 



