Page:Popular Science Monthly Volume 7.djvu/62

52 must, as was formerly supposed, have been directly impressed upon the nervous organization at the creation of the first bee, and transmitted by each succeeding generation, or, as contended by Herbert Spencer and others, the race must have become gradually endowed with it, by a constant repetition of those acts which each individual was stimulated to perform by its surroundings at succeeding times. The former method presupposes a special creation and endowment for each species of animals; a supposition generally rejected by scientific men as presenting insurmountable difficulties, and as not having facts within possible reach to sustain it; for no one has ever known of a special creation. The other method presupposes development in some of its various phases, which, although not without its difficulties, satisfies so many existing conditions, and is constantly helping to solve so many formerly insoluble problems, that scientific men are led to adopt it, provisionally at least, as probably true in its main features, and certainly of great importance as an aid in further investigations.

According to this theory, instinct is the aggregate or accumulated experience of each race of creatures in which it is found—is impressed by repetition upon the nervous organization, and is inherited alike by each individual of a race, causing their actions to be the same, generation after generation, unless changed by necessity from changed surroundings. This is the mode of action characterizing the large class of animals whose highest nervous development is the sensorium. It embraces the cephalous mollusks and the whole division of articulates; and its highest development is reached in insects.

Ascending now another step in the series of animated forms, we find again an advance in the nervous organization suited to the still more complex action and consequent needs of the animal.

Without following out in detail the gradations which take place in passing from the articulate to the vertebrate series, we find in general terms the following changes to have occurred; and, as an example, one of the first and simplest of the series may be taken, namely, the fish. Instead of the ventral cord, with its interrupted series of ganglia, as in the centipede, we find in the fish a spinal cord, existing as a continuous line of ganglionic matter, inclosed in a fibrous sheath of white conducting matter, and the whole protected in the bony canal formed for it by the beautifully-arranged pieces which make up the vertebral column, or back-bone. From this continuous ganglionic centre nerves are given off at different points, as they are needed to supply the different muscles of the body, and especially those of locomotion. We find the sensorium enlarged to preside over the improved organs of special sense, the optic ganglia still being much the more important; and we have two new ganglionic masses added, the cerebrum in front of the optic ganglia, and the cerebellum, placed just behind them. Concerning these, it may be remarked that the latter, although its function has not been so clearly demonstrated as many