CHAPTER IV 



THE DISTRIBUTION OF VARIANTS IN NATURE 



In this chapter we propose to consider the manner in which 

 variations are distributed in nature. As indicated in Chapter I 

 the distribution is not purely random. Groups of various 

 kinds are manifest on the most superficial inspection, and it 

 is our object to describe the various kinds of aggregates found 

 and the mode of their occurrence, and to indicate any general 

 inferences which may be drawn from the latter. 



As a preliminary to this inquiry we have to discuss certain 

 general principles and facts which have an immediate bearing 

 on this subject. 



i. In Chapter II we have given certain data relating to 

 the susceptibility of the living organism to its environment 

 and have discussed how far we can form an opinion as to the 

 likelihood that the effect of such susceptibility is heritable. 

 Apart from the latter all-important question, it is clear that 

 some part of the variation (both in individuals and in popu- 

 lations) in nature is causally related to the factors of the en- 

 vironment. How far we are entitled to consider the characters 

 of any variants and groups as heritable and how far our 

 knowledge is embarrassed by ignorance in this respect will 

 be discussed in 3. 



In addition to the significant and universal occurrence of 

 groups already noted (Chapter I), it is known (Chapter II) 

 that there is another broad principle of distribution of which 

 the essential characteristic is the correlation of some progressive 

 modification or series of modifications with a climatic or 

 environmental ' trend ' or ' gradient.' Such a series is often 

 represented by a number of subspecies or races, as in the 

 subspecies of the Fox Sparrow {Passerella iliaca) of N.W. 

 America (Swarth, 1920). Many cases of single-character 

 modifications are seen in the data brought forward in support 



