ADJUSTMENT OP ORGANISMS TO ENVIRONMENT 381 



motion they were better able to possess the land, and they 

 multiplied in numbers and competition ensued. Super- 

 added to the stress of competition was the direct onslaught 

 of active enemies. Conditions became hard, and various 

 shifts for a living were resorted to. The main lines these 

 shifts could take were determined, however, by environ- 

 ing conditions. There was room to run in, if speed could 

 be attained. There was soil to hide in, if one could dig; 

 there were trees to climb ; there was w^ater to dive in ; 

 and if anything could fly the air offered the best of all 

 ways of escape. 



So land animals differentiated, somewhat as indicated 

 graphically in the accompanying diagram (fig. 222) into 

 cursorial, fossorial, arboreal, aquatic and aerial groups. 



Size. — Owing to the nature of the environment, its 

 limited quantities of food, its limited and irregularly 

 distributed accommodations for shelter, size came early to 

 be a determining factor in the adjustment. For the small 

 animal, while at a disadvantage in point of strength, is at a 

 great advantage when it comes to finding food and shelter. 

 A flag weevil can find a life's provision in one chamber of an 

 iris seed capsule, and leave enough seed untouched to main- 

 tain the plant stock, while an ox may browse to the point of 

 extermination all the herbage on half an acre of ground. 



The kind of differentiation of habitat possible to the larger 

 vertebrates, w^as possible to terrestrial invertebrates upon a 

 smaller scale. The runners, climbers, burrowers, etc., 

 among the beasts of the forest have their counterparts in 

 groups of like habits among the insects of the meadow. 

 Moreover, among the lesser animals that climbed the tree or 

 that went down into the burrow of the beast, there was a 

 secondary, parallel differentiation; so that on the trunk of 

 the tree, and on the walls of the burrow we find small bur- 

 rowing, running, jumping and flying forms Indeed, 



