THE KINSHIP OF LIFE. 33 
degenerate forms. So it is with cave animals, as well as 
with most organisms of the deep sea or the far North. 
All forms which are withdrawn from open competition 
to a solitary and secluded life lose one by one the ad- 
_ vantages which competition has gained for them, and 
are known as degenerate types. What is true of the 
lower animals is likewise true of man. The highest 
type of manhood, of human powers and human virtues, 
will come from victory in the struggle for existence and 
not from withdrawal from the struggle. Easy living 
always brings degeneration. The sheltered life is the 
source of weakness. The desire to get something for 
nothing is the bane of human society. 
Parallel with the case of general degeneration of 
type is that of the degeneration of individual parts of the 
organism. An organ well developed in 
one group of animals or plants may in 
some other be reduced to an imperfect organ or rudi- 
ment so small or incomplete as not to perform its nor- 
mal function, or, indeed, to serve any purpose whatever. 
Such rudimentary or functionless structures may be 
found in the body of any of the higher animals and in 
most or all of the higher plants. The appendix vermi- 
formis and the unused muscles of the ears in man are 
examples, Such are also the atrophied lung, pelvis, and 
limbs of the snake, the “thumb” of the bird, the splint 
bone of the horse, and the like, without mentioning less 
familiar internal organs. By the theory of descent we 
may understand how much structures may be retained 
by the action of the law of heredity, while their reduc- 
tion may be the result of long-continued disuse, or the 
growth and selection of other organs at the expense of 
these which are no longer needed. 
Among a multitude of examples I need refer espe- 
cially to but one—a recent discovery in homology. 
Vestigial organs. 
