SCHMIDT: ANIMAL GEOGRAPHY 787 



Darwin had been in Brazil, by the radical difference in every aspect of land life 

 from that familiar to him in England, and was the more astonished at the obvious 

 similarities between the freshwater animals of tropical America and those of his 

 native country. In The Naturalist in Nicaragua, published in 1874, he points 

 out the fact that bodies of fresh water are in general relatively short-lived, at 

 least in any geologic sense. This, on one hand, snuffs out the variations that 

 develop, while on the other, it puts a premium on the capacity for dispersal. 

 Many kinds of the smaller aquatic organisms have resting stages in which they 

 may dry out and become a part of the dust blown up from a dried lake bottom 

 or river bed. The great uniformity of the smaller animals of the fresh waters 

 of North America and Europe is noteworthy even in the North American Great 

 Lakes, which are no older than the last great advance of the continental glaciers. 



The effects of really long continued isolation in older bodies of water are thus 

 all the more impressive and instructive, with many remarkable side lights on 

 species formation. Lake Baikal in eastern Siberia, the Caspian Sea, Lake Tan- 

 ganyika, and a very few others are preglacial in age, some perhaps dating from 

 the mid-Tertiary; in each of these, animal life has evolved under strict isolation 

 into wonderful series of endemic forms. We are fortunate to have a review of this 

 subject by John Langdon Brooks (1950). To a somewhat lesser extent, the great 

 river systems, like the Mississippi, the Danube, and the Yangtze are also ancient 

 and isolated fresh waters, with strikingly peculiar animal types confined to them. 

 The remnants of former lakes and river systems in old continental arid regions 

 preserve an especially interesting record of origin and of subsequent isolation 

 of their faunas. The phenomena of speciation in the fishes of the desert basins 

 of the western United States have been summarized by Hubbs and Miller (1948). 



Another important advance in the study of the problems of dispersal has 

 been made by analyzing the capacity for adjustment to brackish and salt water 

 by freshwater fishes. It is evident that this capacity is much greater than has 

 been suspected; and it is evident also that the limitation of the distribution of 

 freshwater fishes by salt water barriers is a phylogenetic phenomenon; Myers 

 (1949) formulated these ideas by grouping freshwater fishes according to their 

 capacity for dispersal. His groups are: Primary, strictly intolerant of salt water; 

 Secondary, rather strictly confined to fresh water, but relatively salt tolerant, 

 at least for short periods; Vicarious, presumably nondiadromous freshwater rep- 

 resentatives of primarily marine groups; Complementary, freshwater forms of 

 marine groups, often diadromous, which become dominant in fresh water only 

 when the first three divisions are absent; Diadromous fishes that migrate from 

 fresh to salt water and vice versa; and Sporadic, for fishes that live and breed 

 indifferently in salt or fresh water, without a fixed pattern of migration. 



Isolation and Speciation 



An ecological and evolutionary field of study that has been emphasized since 

 the time of Darwin relates to the first beginnings of the origin of species. Darwin 

 apparently greatly underestimated the role of geographic isolation in the sepa- 

 ration of an incipient species from its parent form or from related forms, and 

 this was first adequately emphasized by Moriz Wagner, the traveler and collector, 

 whose attention was called to changes at the species level from one area to another 



