Page:Evolution of Life (Henry Cadwalader Chapman, 1873).djvu/197

Rh of these animals. The rudimentary condition of the muscles moving the external ear, in domestic animals, is no doubt due to their disuse; domestic animals not being, like wild ones, continually on the watch for prey or enemies. Gymnasts illustrate well the effect of using the muscles, sedentary persons of their disuse. The development of the mental faculties by their use is well seen in the domesticated Dog and Horse as compared with these animals when in a wild condition; while their degeneration through want of use has been noticed in the domesticated Rabbit, whose senses are not so keen as those of the wild one. Parasites are interesting in this respect: thus, the young of many parasites lead a free, active life, exhibiting often a complex organization. In the course of time, however, in adopting a parasitic mode of life they lose many organs, or retain them only in a rudimentary condition. The greater development of the bones, muscles, and nerves of the right hand as compared with the left, is due, no doubt, originally to greater use. This variation, like many others, has been inherited by posterity, since the new-born child offers the same difference in its hands. The thickened skin on the soles of the feet and the palms of the hand (seen also in the new-born child) has no doubt been acquired in the same way. The eye which is most used in microscopy becomes near-sighted, the other eye far-sighted. It must be borne in mind, also, that variations beget variations; the different organs of a plant or animal being so correlated that it is impossible to modify one organ without sooner or later some other organ becoming affected: thus, the increased flow of blood to a part, through continual and violent muscular action, may finally produce hypertrophy of the heart. There are many variations, however, arising through the correlation of organs, which cannot be so readily explained: thus, certain Pigs and Dogs, who lose their hair when taken to warmer climates, have their teeth affected.