Some men are fond of dogs and detest cats; others are devoted to cats and dislike dogs. Inquiring minds have delighted to ask in such cases, Why are cats attractive to one person, dogs to another? Why?
I do not think that this is the most fruitful way of stating the problem. I believe it to be more important to ask in what other respects lovers of dogs and of cats differ from one another. The habit, where one difference has been detected, of seeking for the associated differences, will prove extremely useful not only to pure morphology and to the science of character, but ultimately to physiognomy, the meeting-point of the two sciences. Aristotle pointed out long ago that many characteristics of animals do not vary independently of each other. Later on Cuvier, in particular, but also Geoffrey St. Hilaire and Darwin made a special study of these "correlations." Occasionally the association of the characters is easy to understand on obvious utilitarian principles; where for instance the alimentary canal is adapted to the digestion of flesh, the jaws and body must be adapted for the capture of the prey. But association such as that between ruminant stomachs and the presence of cloven hoofs and of horns in the male, or of immunity to certain poisons with particular colouring of the hair, or among domestic pigeons of short bills with small feet, of long bills with large feet, or in cats of deafness with white fur and blue eyes—such are extremely difficult to refer to a single purpose.
I do not in the least mean to assert that science must be content with no more than the mere discovery of correlations. Such a position would be little better than that of a person who was satisfied by finding out that the placing of a penny in the slot of a particular automatic machine always was followed by the release of a box of matches. It would be making resignation the leading principle of metaphysics. We shall get a good deal further by such correlations, as, for instance, that of long hair and normal ovaries; but these are within the sphere of physiology, not of morphology. Probably the goal of an ideal morphology
