352 Evolution and Adaptation 
perpetuation of the race. In the case of the mammals the 
young are born so immature that they are dependent on 
the parental organs, just spoken of, for their existence., 
Could we follow this relation through its evolutionary stages, 
it would no doubt furnish us with important data, but un- 
fortunately we can do no more than guess how this relation 
became established. The changes in the young and in the 
parent may have been intimately connected at each stage, or 
more or less independent. If we suppose the mammary 
glands to have appeared first, they might have been utilized 
by the young in order to procure food. Their presence 
would then make it possible for the young to be born in an 
immature condition, as is the case with the young of many 
of the mammals. But this is pure guessing, and until we 
know more of the actual process of evolution in this case, it 
is unprofitable to speculate. 
DEGENERATION 
In almost every group of the animal kingdom there are 
forms that are recognized as degenerate. This degeneration 
is usually associated with the habitat of the animal. In many 
cases it can be shown with much probability that these degen- 
erate forms have descended from members of the group that 
are not degenerate. We find there is a loss of those organs 
that are not useful to the organism in its new environment. 
The degeneration may involve nearly the whole organization 
(except as a rule the reproductive system), as seen in the 
tapeworm, or only certain organs of the body, as the eyes in 
cave animals. A few examples will bring the main facts 
before us. 
A parasitic existence is nearly always associated with de- 
generation. Under these conditions, food can generally be 
obtained without difficulty, at the expense of the host, and 
apparently associated with this there is a degeneration, and 
