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 



