54 ELEMENTS OF BIOLOGY. 



sight that the butterflies themselves have been attacked by 

 real fungi." This same eminent observer has pointed out 

 that the walking-stick insects increase their resemblance to 

 twigs and branches by their having the very singular habit 

 of stretching out their legs in an un symmetrical and irregu- 

 lar manner. 



V. CORRELATION OF GROWTH. This term is employed 

 to designate the empirical law that certain structures, not 

 necessarily or usually connected together by any discover- 

 able link, invariably co-exist or are associated with one 

 another, and do not, so far as human observation goes, 

 occur apart. 



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 

 know, and perhaps we shall at some future time find mam- 

 mary glands associated with a single occipital condyle. 

 Again, the feet are cleft in all animals which ruminate, but 

 not in any other. In other cases the correlation is even 

 more apparently lawless, and is even amusing. Thus all, or 

 almost all, cats which are entirely white and have blue eyes, 

 are at the same time deaf. With regard to these and similar 

 generalisations we must, however, bear in mind the follow- 

 ing 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 corre- 

 lations of structure, though they are certainly in no case 

 accidental. 



3. The law is a purely empirical one, and expresses no- 

 thing more than the result of experience j so that structures 



