It was probably during the thirteen years of his second residence at Athens, that Aristotle composed or completed the greater part of his works which have descended to our days. The foundation of most of them was, doubtless, laid at an early period of his life; but they appear to have been gradually formed, and to have received continual additions and corrections. Among the works which especially belong to this period of his life, are his treatises on Natural History; which, as has been correctly observed by a late writer on this subject, are not to be considered as the result of his own observations only, but as a collection of all that had been observed by others, as well as by himself. It is stated by Pliny, that “Alexander the Great, being smitten with the desire of knowing the natures of animals, ordered several thousand persons, over the whole of Asia and Greece, who lived by hunting, bird-catching and fishing, or who had the care of parks, herds, hives, seines, and aviaries, to furnish Aristotle with materials for a work on animals.” We are likewise informed that Aristotle received from Alexander the enormous sum of eight hundred talents,—nearly a million of dollars, to prosecute his researches in natural history,—a circumstance which did not escape the malice of his traducers, who censured him for receiving gifts from princes. Seneca, who states that Philip furnished Aristotle with large sums of money for his history of animals, had, doubtless, confounded the father and son. Callisthenes, a relation of Aristotle, by his recommendation, attended Alexander in his expedition to Asia, and sent from Babylon to the philosopher, in compliance with his previous injunctions, the astronomical observations which were preserved in that ancient city, and which, according to the statement of Porphyrius, reached back as far as 1903 years before the time of Alexander the Great; that is, 2234 years before the Christian era.