This term is employed by zoologists to express the empirical 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.
Fig. 2. - Walking-Leaf Insect (Phyllium).
Thus, all animals which possess two condyles on the occipital 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 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 generalisations 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 more than the result of experience; so that structures which we now only know as occurring in association, may ultimately be found dissociated, and conjoined with other structures of a different character.