Page:Popular Science Monthly Volume 76.djvu/536

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HERE is no doubt, as Jevons has remarked, that if ants had better brains than men, they would either destroy the human race or reduce it to a state of slavery, but these busy little workers offer no black or yellow peril to mankind, for they are all headed in the wrong direction. In the social hymenoptera nature seems to have done her best with a nervous system built upon the simple arthropod plan, in which segmentation, begun at a still lower level in the animal scale, is the dominant character of its structure, and instinct the ruling method of its response.

The vertebrate, on the other hand, has a nervous system of not only a higher but of a very different order, in which response has left the beaten track of instinct, and become more and more molded upon experience. Classification of these higher types on the ground of anatomy agrees plainly with classification on the score of behavior, and this agreement is based upon the structure of the nervous system, the chief function of which is to order and control response.

It is to the evolution of the cerebrum that the vertebrate owes its powers of rational response, and the higher we rise in the scale of vertebrate ascent from the bony fishes, the greater the development of the cerebral cortex, and the keener the mind of the animal, or the greater its power to subdue its hereditary tendencies, to make its acts accord with the results of experience, or the needs of the moment, and to anticipate the future.

The instincts used to be regarded as immutable, and are now often spoken of as "stereotyped," but in the use of the latter term there is need of frequent qualification, and habits even by repetition become automatic. The mechanical operation of habits, which is universally recognized, has given rise to the idea that acquired automatism may appear in the descendants as inherited or congenital automatism, that instincts are inherited habits and merely illustrate "lapsed intelligence," or habits from which the intellectual processes through which they were originally acquired have "lapsed" or disappeared. The confusion and absurdity with which this view can invest a difficult question is well illustrated by Eimer's attempt to explain the parasitic instinct of the European cuckoo, which regularly lays its eggs in other birds' nests.