l6 THE FOUNDATION OF ECOLOGY 



habitat is not so evident. The relation is further obscured by the fact that 

 no physical factor has the direct effect upon them which water or light 

 exerts upon the plant. A^egetation, indeed, as the source of food and pro- 

 tection, plays a more obvious, if not a more important part. This is 

 especially true of anthophilous insects, but it also holds for all herbivorous 

 animals, and, through them, for carnivorous ones. The animal ecology of 

 a particular region can only be properly investigated after the habitats and 

 plant formations have been carefully studied. Here, as in floristics, a great 

 deal can be done in the way of listing the fauna, or studying the life habits 

 of its species, without any knowledge of plant ecology ; but an adequate 

 study must be based upon a knowledge of the vegetation. Although animal 

 formations are often poorly defined, there can be no doubt of their exist- 

 ence. Frequently they coincide with plant formations, and then have very 

 definite limits. They exhibit both development and structure, and are sub- 

 ject to the laws of invasion, succession, zonation, and alternation, though 

 these are not altogether similar to those known for plants, a fact readily 

 explained by the motility of animals. Considered from the above point 

 of view, zoogeography is a virgin field, and it promises great things to the 

 student who approaches it with the proper training. 



24. Sociology. In its fundamental aspects, sociology is the ecology of a 

 particular species of animal, and has in consequence, a similar close con- 

 nection with plant ecology. The ' widespread migration of man and his 

 social nature have resulted in the production of groups or communities 

 which have much more in common with plant formations than do formations 

 of other animals. The laws of association apply with especial force to 

 the family, tribe, community, etc., while the laws of succession are essen- 

 tially the same for both plants and man. At first thought it might seem 

 that man's ability to change his dwelling-place and to modify his environ- 

 ment exempts him in large measure from the influence of the habitat. The 

 exemption, however, is only apparent, as the control exerted by climate, 

 soil, and physiography is all but absolute, particularly when man's depend- 

 ence upon vegetation, both natural and cultural, is called to mind. 



The Es.sentials of a System . 



25. Cause and effect: habitat and plant. In seeking to lay the foundation 

 for a broad and thorough system of 'ecological research, it is necessary to 

 scan the whole field, and to discriminate carefully between what is funda- 

 mental and what is merely collateral. The chief task is to discover, if 

 possible, such a guiding principle as will furnish a basis for a permanent 

 and logical superstructure. In ecology, the one relation which is precedent 



