CORRELATION OF GROWTH. I/ 



sent a singular, and sometimes extremely close, resemblance 

 in their external characters. Thus the composite Hydroid 

 Polypes and the Polyzoa are singularly like one another ; 

 so much so, that they have often been classed together, 

 whereas, in reality, they belong to different sub-kingdoms. 

 Many other cases of this " mimetic " resemblance of different 

 animals might be adduced, and in many cases these "repre- 

 sentative forms " appear to be able to fill each other's places 

 in the general economy of nature. This is so far true, at any 

 rate, that " homomorphous " forms are generally found in dif- 

 ferent parts of the earth's surface. Thus, the place of the 

 Cacti of South America is taken by the Euphorbias of Africa ; 

 or, to take a zoological illustration, many of the different orders 

 of Mammalia are represented in the single order Marsupialia 

 in Australia, in which country this order has almost alone to 

 discharge the functions elsewhere performed by several orders. 

 Many mimetic forms, however, live peacefully side by side, 

 and it is difficult to say whether in this case the resemblance 

 between them is for the advantage or for the disadvantage of 

 either. 



8. CORRELATION OF GROWTH. 



This term is employed by zoologists to express the empi- 

 rical law, that certain structures, not necessarily or usually 

 connected together by any visible link, invariably occur in 

 association 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 : 



i. 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. 



VOL. I. B 



