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acąuisition and resulted directly from the necessity to enter the water for oviposition. The separation of the two families occurred while the adults were as yet only casually aąuatic; crawling into the water to deposit their eggs and carrying with them a supply of air beneath the elytra. This separation was physiological. The larvae from which arose the Dytiscidae spe-cialized for in-shore life and obtained their oxygen directly from the atmosphere: the larva of the pre-gyrinid stock migra-ted to deeper water and, developing tracheal gills, was emanci-pated from the atmosphere. The crowded waters near the shore were the scene of fierce competition; they were also rich in food supplies. The larvae which inhabited them were well supplied with oxygen and were therefore capable of achieving the great activity and rapid growth ratę essential to success in an environment where food is abundant and the „expectation of life” enjoyed by the inhabitants is short. Smali batches of eggs laid at intervals over an extended period were necessitated: large numbers produced at one time resulting in famine. The imago that lived longest and entered the water most freąuently for oviposition served the race best. This would be likely to result in the evolution of swimming. Beneath the water, life, for the imago, was easy. The s wimming insect has few enemies except fish, The common ancestor of the Gyrinidae and the Dytiscidae was probably noxious tasting. Both families today possess strong smelling secretions which are poured out when the insects are seized. Fish reject them and other enemies are few. Food is, of course, abundant. The pressure of natural se-lection consequently fell upon the larva while the imago lived, secure and untroubled by competition, to an advanced age. Thus we find the larvae of the Dytiscidae much perfected, crea-tures of high activity and great ferocity, while the imago has been relatively little modified and retains the Adephagoid facies.

The conditions of life arising from the habits of those larvae which developed aąuatic respiration is almost the reverse of that which affected those specializing for a life in shallow waters. The larva, despite its adeąuate feeding mechanism and the plentiful food supplies of deep waters, could not attain a high metabolic ratę because of the morę limited oxygen supplies available. In the early stages of its evolution, when the