CHAPTER VI 



CORRELATION 



In a previous chapter we have endeavoured to show that 

 throughout the animal kingdom there is a tendency for indi- 

 viduals to be capable of arrangement in a hierarchy of groups, 

 each group being defined by an association of characters 

 which are more or less correlated together. It is evident that 

 whatever the cause or causes of evolution may be, one of its 

 most characteristic effects is the divergence of groups distin- 

 guished by blocks of characters which tend to hang together. 

 Much of this correlation is far from unexpected and calls for 

 little comment. It is not surprising, e.g., that a given mammal 

 of carnivorous habits should have teeth adapted for tearing 

 or crunching, a skull with suitable muscular attachments and 

 limbs appropriate to a raptorial habit. The regular association 

 of characters whose functional significance is far from apparent, 

 such as we see in species and subspecies, is quite another 

 matter and is the main theme of this chapter. 



From a restricted point of view the origin of such correlation 

 appears a relatively simple problem, but a full treatment in- 

 volves the examination of some of the most difficult problems 

 in biology. It is easy to suggest how a group of characters 

 (each regarded as the expression of a single genetic factor) 

 could come to be correlated together, even if we cannot actually 

 verify our hypothesis in any concrete example. There is, 

 however, a tendency to treat the separate characters as some- 

 thing apart from the fundamental organisation of the living 

 animal (cf. Chapter IX). While this may be a justifiable 

 simplification for the practical purposes of genetics and 

 taxonomy, as we shall show at the end of this chapter, it 

 comes into conflict with another conception of the living 

 organism. 



The term correlation has, since Darwin first made the 



