CORRELATION OF GROWTH. I g 



examples of "mimicry," and such animals are said to be 

 " mimetic." Excellent examples of this may be found amongst 

 certain Butterflies, or in the close resemblance of the clear- 

 winged Moths to Bees and Hornets. In all these cases it 

 appears that the mimetic species is protected from some enemy 

 by its outward similarity to the form which it mimics. Finally, 

 there are numerous cases in which animals mimic certain 

 natural objects, and thus greatly diminish their chances of 

 being detected by their natural foes. Excellent instances of 

 this are afforded by the insects known as Walking-leaves 

 (Phyllium) and Walking-sticks (Phasmida), which respectively 

 present the most singular resemblance to leaves and dried 

 twigs. The student, however, must carefully guard himself 

 against supposing that the term "mimicry" implies any 

 conscious action on the part of the mimetic species; there being 

 no evidence to support such a view. 



8. CORRELATION OF GROWTH. 



This term is employed by zoologists to express the empiri- 

 cal law, that certain structures, not necessarily or usually con- 

 nected together by any visible link, invariably occur in associa- 

 tion with one another, and never occur apart so far, at any 

 rate, as human observation goes. 



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 appa- 

 rently 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 gene- 

 ralisations 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. 



3. The law is a purely empirical one, and expresses nothing 



