It is extremely doubtful whether the lowest vertebrates have any power of acquiring mental traits, of varying mentally so as to place themselves in greater harmony with the environment. But, a little higher in the scale, reason certainly appears, and gradually overshadows reflex action and instinct, its growth being synchronous and related to that of the higher portions of the brain, the cerebrum and the cerebellum, especially the former. The lower vertebrates, like the lower insects, and unlike the higher vertebrates and insects (e.g. ants and bees), do not generally tend their young, and therefore can teach them nothing. By virtue of a knowledge which is inborn and hereditary, and which cannot possibly have been acquired, they deposit their eggs under circumstances favourable for survival (e.g. most fish), and by virtue of a similar knowledge, the young when they emerge from the egg are fully equipped mentally for the battle of life. Some fish (e.g. sticklebacks), however, protect their young for a few days after they are hatched, during which time the latter may acquire from their parents such traits as watchfulness against enemies, knowledge of shelter, &c. That little is acquirable by fish is known to every keeper of an aquarium, but that they have some small power of mentally varying, so as to place themselves in harmony with an environment which has become more complex, is proved by such facts as that they may be tamed to a limited extent, and that the wariness of trout increases in a much-fished stream.
Batrachians show distinct powers of acquiring mental traits. Many frogs and toads have been tamed. The following is an extreme case.
