Page:Popular Science Monthly Volume 68.djvu/504

500 while in still deeper water is a golden-yellow form, the original Cephalopholis fulvus. Similar relation of olive and red forms, or of black and yellow types, has been noticed in other groups of fishes.

In similar fashion, it is claimed that within the species lines of segregation may be set up on a physiological basis those of a certain type not breeding freely with those of other types. This theory is largely hypothetical. It is conceivably true under certain circumstances, but there is little evidence that any particular species has originated in that way.

It is beyond question that differences greater than those ordinarily separating distinct species may be produced by continuous conscious or unconscious selection on the part of man. To what extent these breeds would retain their characters under the leveling processes of nature, it is impossible to say in any particular case. On the other hand, in the nature of things, all of them are of very much more recent origin than competing species in nature. While it may be that no wild species has originated from human selection, it is true, on the other hand, that the races thus produced are the same in essence as the subspecies and species produced in nature. The same forces are at work, the basis of selection only being altered by man. The river flows according to the same laws over a natural ledge of rock or over an artificial dam.

Hybridization, as above shown, may produce species as well marked and as fertile as any natural species. But no wild species is known to have arisen from this cause, unless we regard the warbler, Helminthophaga leucobronchialis, as an established species.

A saltation or mutation, beginning with an individual, may extend its characteristics to a numerous progeny, thus forming the beginning of a species. But while this influence may in theory, or even in fact, have a large importance, it is not likely that many species originate in this way. There is no clear evidence that any wild species known to us has arisen from a sudden large individual variation or mutation.

A large number of unexplained, but apparently related, phenomena have been recorded under the name of determinate variation, or have been grouped together as examples of a process to which Eimer has applied the name of Orthogenesis.

Setting theory aside, these cases are essentially of this character. In geological times a certain number of genera appear, each one in a certain direction farther along than its predecessor. Very often a certain organ will be progressively more and more highly developed until a certain point, when it is progressively degraded or simplified in its structure. Examples of this are found among the ammonites, or cephalopods with coiled shells, the chambers in the cell elaborately