Page:Popular Science Monthly Volume 64.djvu/509

Rh but various types of animals became defensive rather than offensive in habit. These include the armored classes, of which the mollusks are the most marked example. To these may be added the forms that seek concealment, either by burrowing or otherwise. These creatures are necessarily sluggish, either from the weight of their armor or their lurking habits. They live upon inactive food, their environment is limited, their contact with nature narrow, their powers of sensation and consciousness little developed. The conditions of their life definitely take them out of the line of the higher progress, in which they can not compete with the more active forms.

In considering then the classes of animals adapted to advanced development, it seems necessary to confine ourselves to the free-moving, agile forms. And among the inhabitants of the ocean—in which life had its origin and its lower stages of development—these are not to be sought among the crawling and burrowing, but among the swimming species. With these the highest activity is dependent upon the most suitable formation of body and the most capable organs of motion.

If we may pursue our fable of nature's experiment in evolution, it can be said that very numerous trials in form were made. There seem possible to colloid substance only two general types of form, the circular or radial and the elongated. Both these were produced in numerous varieties, the circular type embracing two large classes of animals, the cœlenterata and the echinodermata, all of them sluggish, many of them sessile, their general shape and radiated limbs being very ill adapted to active motion. In this respect they were at a great disadvantage as compared with the bilateral, elongated type.

We thus seem to find the experiment of organic evolution, after millions of years of incessant effort, reaching the type which in its simpler stages is popularly designated as the worm, as the form best adapted for advanced evolution. The pristine worm was not in itself a promising creature. Its organs of motion were inefficient and its movements sluggish. Probably several worm-like types appeared, simply organized elongated animals of varied formation, to which we owe, in their final development, the three classes of animals known as the mollusca, the arthropoda, and the vertebrata. This development of an elongated, bilateral animal would seem to have been an inevitable stage in the evolution of animal life, sure to appear in any planet where life had sufficiently progressed, and capable of unfolding into a number of different types. In addition to the great types named, several of minor importance appeared upon the earth, and different ones may well have arisen elsewhere.

Yet if we seek for the highest class of form likely to arise from the worm-like unit, our field of search is restricted. If activity and flexibility of body are advantageous, we must seek these in the swimming rather