Having, after careful study of this Treatise, been led to the conclusion that Aristotle's object, in its composition, was to put before the world his own opinions as well as those of former and contemporaneous writers upon the Vital Principle, I have been induced to undertake a translation of it, in order to give the general reader the theories, hypotheses, and opinions which prevailed, at that early period of natural and physiological knowledge, upon life and its manifestations. The Treatise, indeed, records all the prevailing opinions upon living beings and sentient properties, which lie scattered through Aristotle's other physiological writings; and it displays, perhaps more than any other of his works, the extent of his knowledge, and the perspicacity of his intellect. Should it, however, be questioned whether a work, composed at a time when the special sciences pertaining to its subject were yet in their infancy, can be now of any value, it might be answered that, irrespective of any positive result, an interest must ever be taken in the investigation, truthfully conducted, of nature's operations; and that this, brief
