Aristotle's biology characters, not common to the species, are not due to a final cause; that is to say, they are not useful or conducive to the end which is the life of the animal. " For whenever things are not the product of Nature working upon the animal kingdom as a whole, nor yet characteristic of each separate kind, then none of these things is such as it is or is so developed for any final cause. The eye, for instance, exists for a final cause, but it is not blue for a final cause unless this condition be characteristic of the kind of animal." In other words, when a character is common to all animals of an established group, then it exists for a purpose; but fluctuating characters are not so developed. Such characters have " no connection with the essence of the animal's being, but we must refer the causes to the material and the motive principle or efficient cause, on the view that these things come into being by necessity." Apparently Aristotle means that the formal or final cause cannot always control the material and the efficient causes, and variations from the perfect type arise.*^ Every animal with its essential or constant
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