514 POPULAR SCIENCE MONTHLY. 



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 liv- 

 ing 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 ma- 

 rine 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 descend- 

 ants 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 



