16 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. Vegetation, 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 EssENTIALS 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 
