VII 



How Does Mating Bring About Both Biparental Inherit- 

 ance and Diversity in Hereditary Characters? What Effect 

 Has Mating on the Stock as a Whole? Does It Increase 

 Variation? Does It Decrease Variation? What Is Its 

 Relation to Evolution? 



I N our last chapter we showed that mating produces bi- 

 parental inheritance, as well as diversity of inherited 

 characteristics, in lower as well as in higher organisms. 

 How are these results brought about? How does it happen 

 that the offspring of the two members of a pair on the whole 

 resemble each other, yet are hereditarily diverse? 



The main outlines of the way this is brought about are 

 well known. Each parent hands on bodily to the offspring, 

 through the germ cells, certain packets of chemicals. Since 

 these are directly transmitted from parent to offspring, 

 while the later characters are secondarily derived from 

 them, we may call these packets of chemicals the primary 

 hereditary characters. These packets are present in each 

 animal in a certain definite number, stored within the 

 nucleus; they are called chromosomes (see Figures 29, 31 

 32). Individuals which get different sets of packets from 

 their parents develop differently even under the same outer 

 conditions; that is, they show different hereditary char- 

 acteristics. 



This arrangement of the primary hereditary characters 

 the chemicals that determine the hereditary peculiarities 



170 



