Speusippus, Plato's successor in the Academy, was both philosopher and naturalist, and we may take it, if we please, that his leaning towards biology, and the biological trend which at this time became more and more marked in Athenian philosophy, were not unconnected with the great impulse which Aristotle had given. However this may be, Speusippus wrote a book περι Ὁμοιων {peri Homoiôn} 'Concerning Resemblances'; and this, of which we only possess a few fragmentary sentences, must have been a very curious and an interesting book. He mentions, among other similar cases, that our little fish _phycis_ has a close outward semblance to the sea-perch; and this is enough to clinch the proof that Aristotle's nest-building fish was not a goby but a wrasse. The whole purport of Speusippus's book seems to have been to discuss how, or why, with all Nature's apparently infinite variety, certain animals have a singularly close resemblance to certain others, though they be quite distinct in kind. It is a problem which perplexes us still, when we are astonished and even deluded by the likeness between a wasp and a hover-fly, a merlin and a cuckoo. In certain extreme cases we call it 'mimicry', and invoke hypotheses to account for this 'mimetic' resemblance; and those of us who reject these hypotheses must fain take refuge in others, as far-reaching in their way. This at least we know, that Speusippus seized upon a real problem of biology, of lasting interest and even of fundamental importance.