GREEK BIOLOGY AND MEDICINE known to him for the most part — but not altogether, since now and then the modern investigator " discovers " what Aristotle knew. Yet whole provinces of the considerations of modern biology scarcely touched him. All the more marvellous were the forward thrusts of his mind toward what the distant future should make clear. One of those thrustings forward was the classification of animals, which may be drawn from his writings. His fundamental division was into animals with blood and animals without, that is to say, those who have no true blood but a different fluid performing a like nutritive function. This division coincides with the modern one into vertebrates and invertebrates, ascribed to Lamarck (i 744-1 829). Through the constit- uent groups under both divisions will be found a series of gradations in foetal development within the parent's body; and these determine the Aristotelian group formation. Man, the Cetacea, viviparous quadrupeds, birds, reptiles, fishes, bony and then cartilag- inous, come within the division of animals with blood. Aristotle had no conception of the mammalian ovum, and consequently regarded the embryo of mammals as born alive within
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