Before any experience of parturition, the knowledge that it is approaching drives all mammals into solitude, and bids them prepare a nest for their young in a hole or in some other place of shelter. The bird builds her nest as soon as she feels the eggs coming to maturity within her. Snails, land-crabs, tree-frogs, and toads, all of them ordinarily dwellers upon land, now betake themselves to the water; sea-tortoises go on shore, and many saltwater fishes come up into the rivers in order to lay their eggs where they can alone find the requisites for their development. Insects lay their eggs in the most varied kinds of situations,—in sand, on leaves, under the hides and horny substances of other animals; they often select the spot where the larva will be able most readily to find its future sustenance, as in autumn upon the trees that will open first in the coming spring, or in spring upon the blossoms that will first bear fruit in autumn, or in the insides of those caterpillars which will soonest as chrysalides provide the parasitic larva at once with food and with protection. Other insects select the sites from which they will first get forwarded to the destination best adapted for their development. Thus some horseflies lay their eggs upon the lips of horses or upon parts where they are accustomed to lick themselves. The eggs get conveyed hence into the entrails, the proper place for their development,—and are excreted upon their arrival at maturity. The flies that infest cattle know so well how to select the most vigorous and healthiest beasts, that cattle-dealers and tanners place entire dependence upon them, and prefer those beasts and hides that are most scarred by maggots. This selection of the best cattle by the help of these flies is no evidence in support of the conclusion that the flies possess the power of making experiments consciously and of reflecting thereupon, even though the men whose trade it is to do this recognise them as their masters. The solitary wasp makes a hole several inches deep in the sand, lays her egg, and packs along with it a number of green maggots that have no legs, and which, being on the point of becoming chrysalides, are well nourished and able to go a long time without food; she packs these maggots so closely together that they cannot move nor turn into chrysalides, and just enough of them to support the larva until it becomes a chrysalis. A kind of bug (_cerceris bupresticida_), which itself lives only upon pollen, lays her eggs in an underground cell, and with each one of them she deposits three beetles, which she has lain in wait for and captured when they were still weak through having only just left off being chrysalides. She kills these beetles, and appears to smear them with a fluid whereby she preserves them fresh and suitable for food. Many kinds of wasps open the cells in which their larvæ are confined when these must have consumed the provision that was left with them.