animal life. One of these is a sufficient supply of food; a second, a sufficiency of oxygen; a third, proper nutritive and excretory organs; a fourth, proper reproductive powers.
The first and fourth are of most importance in this connection, for they are in constant conflict with each other. The quantity of available food is far more limited than are the possibilities of animal increase. Necessarily, then, the latter is restricted by the former. A crowding out process ensues, and only those best adapted to obtain food survive.
But an equally necessary result is an adaptation to new sources of food, one of whose earliest consequences is the production of carnivorous animals. Thus the crowding-out process becomes, in part, an eating-out process. The animals thus exposed to destruction would necessarily be at a marked disadvantage in the race for life, were not some protection provided them. For safety they need weapons of defense or means of escape. It all comes to this, then, that the strong. est, swiftest, best-armed, and most alert animals will survive, these qualities enabling herbivora to escape their foes, carnivora to overcome their prey.
But there are two ways in which this survival may be attained: one by adaptation to a few simple conditions; the other by adaptation to many and complex conditions. The wider the scope of adaptation in an animal, the greater is its functional complexity, and the higher its organic position, as compared with the more simplified tribes.
Still another requisite of the utmost importance is the principle of division of labor. No organ can do two distinct things equally well. If forced to perform two or more labors, there must be a degree of imperfection in its work, or its ability in each direction must be greatly limited. Therefore evolution is in the direction of separation of labor, each organ tending to become confined to one kind of work, to which alone it becomes adapted, but in which it produces better and wider results.
Seeking, then, for the features likely to distinguish the most highly developed animal, we may safely say that they will appear in that animal exposed to the most complex conditions, adapted to the greatest variety of food, possessed of the most fully specialized organs, capable of using its innate forces to the best advantage, and commencing its individual existence with the best start in life.
With this preliminary we may proceed to a closer investigation. Vertebrate animals occupy every kingdom of nature—the sea, the land, and the air—but not under equally advantageous conditions. The inhabitants of the sea, for instance, are exposed to decided disadvantages, and lack certain important incentives to development. Their vital activity is necessarily much below that of land-animals, from the limited quantity of oxygen obtainable by water-breathers as compared with air-breathers. Their sensory acuteness, also, is less developed. Light comes to them dimmed, sound comes to them dulled, the
