NATURAL SELECTION. 
147° 
ofthese 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. 
— 
Gym- 
nasts illustrate well the effect of using the muscles, seden- 
tary 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 develop- 
ment 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 
impóssible 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. 
