784 THE POPULAR SCIENCE MONTHLY. 



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, projier 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 availa- 

 ble 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 carnivo- 

 rous 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 over- 

 come 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 j)roduces 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 ani- 

 mal exposed to the most complex conditions, adapted to the greatest 

 variety of food, possessed of the most fully specialized organs, capa- 

 ble 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 in- 

 habitants of the sea, for instance, are exposed to decided disadvan- 

 tages, and lack certain iinportant incentives to development. Their 

 vital activity is necessarily much below that of land-animals, from the 

 limited quantity of oxygen obtainable by w^ater-breathers as com- 

 pared with air-breathers. Their sensory acuteness, also, is less devel- 

 oped. Light comes to them dimmed, sound comes to them dulled, the 



