XVI 

 HEREDITY IN MAN 



1. Having touched upon the influence of the environment in 

 producing modifications, we have now to approach the subject 

 of the selection of characters, both mental and physical. It is 

 proposed to consider in this chapter what it is that is inherited, 

 and in the two following chapters to consider the results of 

 selection. 



What is meant by saying that any character is inherited is clear 

 from former chapters. There are certain predispositions in the 

 germinal constitution of every individual — predispositions to give 

 rise to certain characters under certain conditions and to other 

 characters under other conditions. There is a predisposition, for 

 instance, in the species of primrose referred to in Chapter XIV to 

 give rise to red flowers at a certain temperature and to white 

 flowers at a different temperature, the colour of the flower being 

 equally inherited in both cases. We may therefore take as our 

 starting-point the fact that there are in the constitution of every 

 individual many such predispositions. 



From the results of breeding experiments upon Mendelian lines 

 certain conclusions have been reached which carry us farther. 

 These experiments have shown that certain unit-characters can 

 be isolated which behave in a certain manner when crossed, and 

 from the manner in which they behave it is deduced that they 

 depend upon unit-factors in the germinal constitution which are 

 invariable. The number of unit-characters which have been 

 detected in any one species is not large ; it is true that in one 

 species of insect about two hundred unit-factors have been found, 

 but even this number can only form a small proportion of the 

 total number if it is the case that the germinal constitution 

 consists entirely of unit-factors which on crossing give the Mende- 

 lian ratio. This is the working hypothesis which is put forward 

 as the explanation of inheritance, and which it is the object of 

 investigators now to test. We may consider for a moment what 

 it is that this hypothesis implies. 



