Page:Popular Science Monthly Volume 63.djvu/518

514 hint of the multitudinous minor lines of adaptation, the following of which results in limitation.

Such forms of insects, crustaceans, worms, etc., as have taken to living under bark and wood and become so flattened as conveniently to slip between the wood and bark in crevices scarcely permitting the passage of a sheet of paper—mollusks which bore into piles or some into solid rock—barnacles that fasten to ships and thus reach all climes—insects that thrive in such unnatural food material as tobacco, insect powder; or larvae that live in brine vats, wine vats, or, perhaps most extreme, in crude petroleum. Vinegar eels in vinegar and hosts of forms that subsist upon hams, bacon, flour, meal and other prepared foods, boots, shoes and other leather goods, and furs show the readiness with which new habits are acquired. But these habits are not so easily broken, and it is doubtful if the cheese maggot, having taken up its particular dietary furnished by man, could return to the food of its ancestors before cheeses were made.

I have spoken in places of the choice of animals for a particular sphere as if this were a conscious element, and I do not care to dispute those who maintain some such element as operative in the mutations of animal life. But, conscious or unconscious, it appears to me that the animal, even in a lowly sphere, has the power to choose in some degree or other the direction of its activity, to put itself in certain environments, and while not selecting certain changes of structure by the mere fact of selecting such environment, submits itself to inevitable changes which that environment must perforce produce. Thus, an animal may not elect to become flattened in body, but selecting narrow quarters in which flattening is necessary or advantageous this change is sure to follow.

The ancestors of snakes may not have determined upon eliminating legs from their anatomy, but by choice of habitat where legs were in the way these organs were gradually reduced and lost. But by no process of electing locations where legs are helpful can we expect the animal to restore the organ thus sacrificed. The fly that mimics a bee may never in its ancestral line have started out to make itself resemble a bee, but it may have chosen such relation to bee life that a similarity became distinctly advantageous or even necessary, and then natural selection could clearly come into operation to produce the mimicry. The ancestral whale had no glimmer of thought of launching into marine life, to trade his feet for paddles and flukes, when he first by virtue of some advantage found entrance to water desirable, but once the way was entered, selection and environment conspired to carry his descendants further and further along a path which knows no backward steps.

What fearful consequences attend these unconscious selections! Shall it be persistence or progress, mere survival or advancement—the