Aristotle's voluminous writings have come down to us through many grave vicissitudes. The greatest of them all are happily intact, or very nearly so; but some are lost and others have suffered disorder and corruption. The work known as the 'Parts of Animals' opens (as our text has it) with a chapter which seems meant for a general exordium to the whole series of biological treatises; and I know no chapter in all Aristotle's books which better shows (in plainer English or easier Greek) the master-hand of the great Teacher and Philosopher. He begins by telling us (it has ever since been a common saying) that every science, every branch of knowledge, admits of two sorts of proficiency--that which may properly be termed scientific knowledge, and that which is within the reach of ordinary educated men. He proceeds to discuss the 'method' of scientific inquiry, whether we should begin with the specific and proceed to the general, or whether we are to deal first with common or generical characters and thereafterward with special peculiarities. Are we entitled to treat of animals, as is done in mathematical astronomy, by dealing first with facts or phenomena and then proceeding to discover and relate their several causes? At once this leads to a brief discussion (elaborated elsewhere) of the two great Causes, or aspects of cause--the final cause and the 'moving' or efficient cause--the _reason why_ or the purpose for which, and the antecedent cause which, _of necessity_, brings a thing to be such as it is. Here is one of the great crucial questions of philosophy, and Aristotle's leaning to the side of the Final Cause has been a dominant influence upon the minds of men throughout the whole history of learning. Empedocles had taken another view: he held that the rain comes when it listeth, or 'of necessity'; that we have no right to suppose it comes to make the corn grow in spring, any more than to spoil the autumn sheaves: that the teeth grow by the operation of some natural (or physical) law, and that their apparent and undoubted fitness for cutting and grinding is not purposeful but coincident; that the backbone is divided into vertebrae because of the antecedent forces, or flexions, which act upon it in the womb. And Empedocles proceeds to the great evolutionary deduction, the clear prevision of Darwin's philosophy, that fit and unfit arise alike, but that what is fit to survive does survive and what is unfit perishes.