The philosophical bases of Aristotle’s biology are mainly to be found in the treatise _On soul_ and in that _On the generation of animals_. His actual observations are contained in this latter work—which is in many ways his finest scientific production—in the great collection on the _History of animals_, and in the remarkable treatise _On parts of animals_. Certain of his deductions concerning the nature and mechanism of life can be found in his two works which deal with the movements of animals (one of which is very doubtfully genuine) and in his tracts _On respiration_, _On sleep_, &c. The treatise _On plants_ and the _Problems_ in their present form are late and spurious, but they are based on works of members of his school. They were, however, perhaps originally prepared at the other end of the Greek world in Magna Graecia. Aristotle was a most voluminous author and his biological writings form but a small fraction of those to which his name is attached. Yet these biological works contain a prodigious number of first-hand observations and it has always been difficult to understand how one investigator could collect all these facts, however rapid his work and skilful his methods. The explanations that have reached us from antiquity are, indeed, picturesque, but they are neither credible in themselves nor are they consistent with each other. Thus Pliny writing about A. D. 77 says ‘Alexander the Great, fired by desire to learn of the natures of animals, entrusted the prosecution of this design to Aristotle.... For this end he placed at his disposal some thousands of men in every part of Asia and Greece, and among them hunters, fowlers, fishers, park-keepers, herds-men, bee-wards, as well as keepers of fish-ponds and aviaries in order that no creature might escape his notice. Through the information thus collected he was able to compose some fifty volumes.’[14] Athenaeus, who lived in the early part of the third century A. D., assures us that ‘Aristotle the Stagirite received eight hundred talents [i.e. equal to about £200,000 of our money] from Alexander as his contribution towards perfecting his _History of Animals_’.[15] Aelian, on the other hand, who lived at a period a little anterior to Athenaeus, tells us that it was ‘Philip of Macedon who so esteemed learning that he supplied Aristotle with ample funds’ adding that he similarly honoured both Plato and Theophrastus.[16] [14] Pliny, _Naturalis historia_, viii. 17. [15] Athenaeus, _Deipnosophistae_, ix. 58. [16] Aelian, _Variae historiae_, iv. 19.