VARIATIONS IN ANIMALS AND PLANTS 499 



and the evidence in favor of interoeeanic connection is not weakened by an 

 increase in the one list at the expense of the other. All evidence concurs in 

 fixing the date of that connection at some time prior to the Pleistocene, prob- 

 ably in the early Miocene. When geological data shall be adequate definitely 

 to determine that date, it will give us the best known measure of the rate of 

 evolution in fishes. 



Of the 82 families of fishes represented at Panama, all but three (Cerdalidae, 

 Cirrhitidae and Nematistiidse) occur also on the Atlantic side of Central Amer- 

 ica, while of the 218 genera of the Panama list, no fewer than 170 are common 

 to both oceans. The well-developed families, Centropomidse and Dactyloscopidae, 

 are peculiar to the two tropical faunas now separated by the Isthmus of Panama. 

 It might be added that the families of Nematestudae (one species) and Cer- 

 dalida? (three species) are confined to the Panama region, while the Cirrhitidae 

 (one species) belongs to a group characteristic of the islands of Polynesia. 



From this discussion, it is probable that even in isolation some 

 species change very slowly, that with similar conditions the changes 

 within isolated groups of a species may be parallel, and that the specific 

 changes in different groups may progress with very different degrees 

 of velocity. Natural selection apparently furnishes the motive power 

 of change, but the initiative comes from variation and heredity, and 

 its direction and final results depend on a multitude of conditions and 

 circumstances of environment which are largely geographical, topo- 

 graphical or climatic in their nature. 



Topographical Segregation 



Topographical segregation may bring about the separation of sub- 

 species or species in precisely the same manner as other methods of 

 geographical isolation. An example is that of the deep-water trout of 

 Lake Tahoe, Salmo henshawi tahoensis. The ordinary Tahoe trout, 

 Salmo henshawi, lives in the shallow parts of the lake, spawning in the 

 streams. This form, larger in size, more robust in form and less 

 spotted in color, lives in the depths of the lake, spawning near the 

 shore. The difference between the two is not great, but is perhaps 

 sufficient to justify the subspecific name (tahoensis). The two are 

 considered as different species by anglers. 



A more strongly marked case, probably of earlier origin, is seen in 

 several West Indian species of grouper or sea bass, belonging to the 

 genus Mycteroperca. In these species, the shore forms have an olive- 

 green color, while others, essentially similar, in deep water are crimson 

 or scarlet. Thus Mycteroperca venenosa, the yellow-fin grouper, has 

 a scarlet cognate form, Mycteroperca venenosa apua, Mycteroperca 

 tigris, likewise green in shallow water, has its deep water representa- 

 tive in M. tigris camelopardalis. The same condition holds with 

 Mycteroperca olfax of the Galapagos and its cognate M. olfax ruber- 

 rima. In another species of this type, the Guativere, Cephalopholis 

 fulvus of the West Indies, the shore form is dark olive, (C. fulvus 

 punctatus) , that found in deeper water is crimson (C. fulvus ruber), 



