CHAPTER IV. 

 INHERITANCE. 



Nothing is more familiar than the close adherence of 

 offspring to the specific type of their ancestry. Although 

 variations abound, they occur within very narrow limits. 

 The egg of a frog can produce only a frog; never a newt, or a 

 salamander. A hen sitting on duck's eggs can never avail 

 to hatch anything but ducklings out of them; for there is 

 nothing else in them. Moreover, our confident expectation 

 that offspring will resemble not only their race, but their 

 individual ancestors as well is expressed by the proverb, 

 "Like father, like son." 



Heredity and variation are two aspects of evolution as 

 viewed from the standpoint of the present, heredity looking 

 toward the past, and variation toward the future. But 

 whether a valuable variation counts for anything or not in 

 racial development depends, as we have seen, upon whether 

 it is heritable or not. ' Hence we must ask, whether able to 

 answer or not, what is the nature of the bond between the 

 generations? Such facts as have been accumulated bearing 

 on this question may be briefly considered under two heads: 

 i) the visible mechanism, and 2) the observable results of 

 heredity. 



I. THE VISIBLE MECHANISM OF HEREDITY. 



The process of reproduction is one of the chief distinguish- 

 ing phenomena of living things. We have in the preceding 

 pages considered numerous remarkable structures and 

 developments connected with it. But to distinguish its 

 essentials we must now retrace our steps and consider again 

 the simpler organisms. The yolk accumulation, the em- 

 bryonic membranes, the milk glands, etc., which we have 



