must have been reached, as a stage superior to the minute single-celled animal, or the immobile plant, fed with inorganic nutriment.
If we may then accept it as inevitable that organic evolution everywhere, if sufficiently advanced, must have reached the stage of the metazoon animal, this may be taken as the necessary basis of higher progress in any life-bearing planet. In the metazoon we have a creature consuming organic food, which it is necessary to seek, and thus needing powers of self-motion, either of the body as a whole or of its members. And in any planet in which beings equivalent to man appeared the faculty of consciousness must have been equally necessary at an early stage, as a highly advantageous aid in the struggle for existence.
This type of life once attained, it formed a fertile field for the operation of the principle of natural selection. Upon the earth, and presumably everywhere, it developed into innumerable forms, each adapted to some passing or permanent condition of the environment. Assuming that the agencies of internal organic activity were everywhere much the same—including active chemical change, due to oxidation or something similar, vascular organs for the conveyance of nutriment to the wasting tissues, apparatus for sensation and motion, and the like—and that these led to the development of specialized organs equivalent to the lungs, the heart, the brain, etc., we shall confine ourselves here to the subject of variation in outward form and condition.
Even in this there are a multitude of relations to consider, and we can deal here only with those of general character. A main one is that of activity as contrasted with inactivity. Many of the new forms became sessile animals, their only active parts being tentacles or other organs of offense and defense. Others became free-moving animals. Of the two types the latter was evidently the best adapted to high development, both physical and mental, its free motion greatly diversifying its environment and bringing it into much more varied relations than could be enjoyed by the plant-like sessile forms. The more active the animal, the more diversified its powers of motion, the more acute and varied its organs of sense, the more alert its powers of consciousness, the higher seemingly would be its position in the ranks of life and the superior its opportunities for upward progress. And this rule must have prevailed not only on the earth, but throughout the universe.
This being the case, not alone the sessile, but the sluggish, forms were at a disadvantage as compared with the active. Anything, then, likely to prevent rapidity and diversity of motion must have acted as a check to progress. Activity is essential to the most effective offensive powers, and upon these the higher stages of development depend;
