< Page:Popular Science Monthly Volume 68.djvu
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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, probably 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 (Cerdalidæ, Cirrhitidæ and Nematistiidæ) occur also on the Atlantic side of Central America, while of the 218 genera of the Panama list, no fewer than 170 are common to both oceans. The well-developed families, Centropomidæ and Dactyloscopidæ, are peculiar to the two tropical faunas now separated by the Isthmus of Panama. It might be added that the families of Nematestudæ (one species) and Cerdalidæ (three species) are confined to the Panama region, while the Cirrhitidæ (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, topographical or climatic in their nature.

Topographical Segregation

Topographical segregation may bring about the separation of subspecies 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 tahoënsis. 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 (tahoënsis). 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 olivegreen 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 representative in M. tigris camelopardalis. The same condition holds with Mycteroperca olfax of the Galapagos and its cognate M. olfax ruberrima. In another species of this type, the Guatívere, 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),

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