In the waters of the globe the predominance of Insect life disappears. In the smaller collections of fresh water many Insects find a home during a portion of their lives, and some few contrive to pass their whole existence in such places; but of the larger bodies of fresh water they invade merely the fringes, and they make only the feeblest attempt at existence in the ocean; the genus _Halobates_ containing, so far as we know, the sole Insects {84}that are capable of using the ocean as a medium of existence at a distance from the shore. It will probably be asked, how has it come about that creatures so insignificant in size and strength have nevertheless been so successful in what we call the struggle for existence? And it is possible that the answer will be found in the peculiar relations that exist in Insects between the great functions of circulation and respiration; these being of such a nature that the nutrition of the organs of the body can be carried on very rapidly and very efficiently so long as a certain bulk is not exceeded. Rapidity of growth is carried to an almost incredible extent in some Insects, and the powers of multiplication—which may be considered as equivalent to the growth of the species—even surpass the rapidity of the increase of the individual; while, as if to augment the favourable results attainable by the more usual routine of the physiological processes, "metamorphosis" has been adopted, as a consequence of which growth and development can be isolated from one another, thus allowing the former to go on unchecked or uncomplicated by the latter. A very simple calculation will show how favourable some of the chief features of Insect life are. Let it be supposed that growth of the individual takes time in proportion to the bulk attained, and let A be an animal that weighs one ounce, B a creature that weighs ten ounces, each having the power of producing 100 young when full grown; a simple calculation shows that after the lapse of a time necessary for the production of one generation of the larger creature the produce of the smaller animal will enormously outweigh that of its bulkier rival. Probably it was some consideration of this sort that led Linnaeus to make his somewhat paradoxical statement to the effect that three flies consume the carcase of a horse as quickly as a lion.[16]