CORRELATION OF GROWTH. 25 



Thus, all animals which possess two condyles on the occi- 

 pital bone, and possess non-nucleated red blood-corpuscles, 

 suckle their young. Why an animal with only one condyle 

 on its occipital bone should not suckle its young we do not 



Fig. 2. Walking-Leaf Insect (Phyllium). 



know, and perhaps we shall at some future time find mammary 

 glands associated with a single occipital condyle. Most of 

 these correlations are physiologically difficult of explanation, 

 and sometimes even amusing. Thus all, or almost all, male 

 cats, which are entirely white and have blue eyes, are at the 

 same time deaf. With regard to these and similar generali- 

 sations we must, however, bear in mind the following three 

 points : 



1. The various parts of the organisation of any animal are 

 so closely interconnected, and so mutually dependent upon 

 one another, both in their growth and development, that the 

 characters of each must be in some relation to the characters 

 of all the rest, whether this be obviously the case or not. 



2. It is rarely possible to assign any reason for correlations 

 of structure, though they are certainly in no case accidental. 



