We see from this example that it is not some inward necessity which thus, in the higher and more complicated organism, contracts the ontogeny into the embryonic state, but that this depends upon external adaptive factors. Here again we have adaptation, mainly to the conditions of larval life. The elimination of the larvæ by enemies, for instance, will, other things being equal, be so much the more incisive the longer the larval development is protracted, but in that case the general ratio of elimination of the species, and the degree of fertility the species must possess in order to hold its own in the struggle for existence, will also play a part in determining the mode of development. For the higher the ratio of elimination the more eggs the female must produce, and the more eggs that have to be produced the smaller will be the quantity of nutritive material for the building up of the young embryo which each egg can be furnished with. I know of no records in regard to the eggs of that Brazilian shrimp in which embryonic development ends with the nauplius stage, but we shall certainly not be wrong in predicting that the eggs in this case will be very small and very numerous, in contrast to those of the freshwater crayfish, which are large and, as compared with others known to us, not very numerous. It is a point of undeniable theoretical significance which the life-histories of these Crustaceans disclose, that embryogenesis is not condensed according to hidden internal laws when the structure increases in complexity, but that the condensation of the ontogenetic stages depends upon adaptation, and may be quite different in nearly related species. It shows us anew that all biological occurrences are dominated by the process of selection.