[187] It may be useful to point out the following examples of parental care among animals in which, as a rule, the eggs are left to take care of themselves. It will be found that in general this instinct is associated with high zoological rank (best exemplified by Mammals and Birds), land or freshwater habitat, reduced number of eggs, and direct development. AMPHIBIA.--The eggs are sometimes hatched by the male (_Alytes obstetricans_, _Rhinoderma Darwinii_), or placed by the male in pouches on the back of the female (_Pipa dorsigera_, _Notodelphis ovifera_, _Nototrema marsupiatum_), or carried during hatching by the female (_Polypedates reticulatus_). FISHES.--The Stickleback and others build nests. Of eleven genera of nest-building Fishes, eight are freshwater. The number of eggs is unusually small. Many Siluroids have the eggs hatched in the mouth of the males, a few under the belly of the female. The species are both marine and freshwater, the eggs few and large. Lophobranchiate fishes usually have the eggs hatched by the male. They are marine; the eggs few and large. Many sharks hatch their eggs, which are very few, within the body. _Mustelus lævis_ has a placenta formed out of the yolk-sac. INSECTS.--De Geer has described the incubation of the Earwig, and the care of the brood by the female. The cases of the social Hymenoptera, &c., are universally known. SPIDERS.--The care of the female spider for her eggs is well known. CRUSTACEANS.--The Crayfish hatches and subsequently protects her young. _Mysis_, _Diastylis_ (_Cuma_), and some Isopods hatch their eggs. _Gammarus locusta_ is followed about by her brood, which shelter beneath her when alarmed. _Podocerus capillatus_ builds a nest among corallines. Several of the _Caprellidæ_ hatch or otherwise protect their young. All these, except the Crayfish, are marine; the eggs commonly fewer than usual. ECHINODERMS.--Many cases of “marsupial development” have been recorded in the species of the Southern seas. Here development, contrary to the rule in Echinodermata, is direct. But what has all this to do with habitat? Are fluviatile and terrestrial animals, as a rule, better off than marine animals? Possibly they are. In the confined and isolated fresh waters at least, the struggle for existence is undoubtedly less severe than in the waters of the sea. This is shown by the slow rate of change in freshwater types. Many of our genera of land and freshwater shells date back at least as far as to Purbeck and Wealden times, while our common pond-mussel is represented in the Coal Measures. The comparative security of fresh waters is probably the reason why so many marine fishes enter rivers to spawn.