40 ENVIRONMENT OF VERTEBRATE LIFE, ETC. 



Moreover, animals are restricted by barriers as obscure as bacterial 

 disease. Many forms of life are actually barred from regions in Africa 

 where the tse-tse fly carries the trypanosome of sleeping sickness, and other 

 parasites are almost equally effective. Man himself is only slowly winning 

 past the barriers of tropical diseases to a vigorous health and growth in the 

 intertropical regions. 



Salt water is an effective barrier to amphibians, as it is fatal to the egg 

 or adult of almost all forms. 



(f) ENVIRONMENT. 



Environment is the sum of all the contacts which an organism or a group 

 of organisms establishes with the forces and matter of its surroundings, 

 either organic or inorganic. With this the concept of isolation becomes 

 much more complex. Complete isolation is unthinkable, but partial and 

 effective isolation may be achieved by the acquisition of certain habits, 

 certain physiological peculiarities or immunities, certain morphological char- 

 acters, etc., which remove a form from a given number of contacts or neutral- 

 ize their effect. Isolation is no longer to be thought of as accomplished 

 solely by the presence of physical barriers. An individual or group which 

 has developed immunity from a contagious disease may continue and exist 

 in a state of isolation from a set of contacts which control the development 

 of individuals or groups around it; physical peculiarities, habits, armor, etc., 

 might have the same effect. 



Such a state of isolation may amount to very perfect adaptation to the 

 environment, and, as the author has suggested, 1 may lead to extinction. 



Any attempt at an analysis of the environment as thus conceived will 

 at once lead to its separation into two main groups, the organic and the 

 inorganic, both of which are susceptible to minute subdivision, and all sub- 

 divisions will show innumerable instances of a most complex interrelation- 

 ship. At the same time, the environment may be divided in a tripartite 

 manner into those contacts which are favorable to the organism or group, 

 those which are unfavorable, and those which are neutral or have no effect. 

 The latter class will inevitably be very small, for so intimate are the inter- 

 relationships of all the forces and matter which surround any unit that the 

 alteration of even the seemingly most negligible factor may have a far- 

 reaching effect upon the whole. In tabular form such an analysis may be set 

 forth as follows: 



Organic, favorable (hospitable). Inorganic, favorable (hospitable) 



Organic, unfavorable (inhospitable). Inorganic, unfavorable (inhospitable). 



Organic, neutral. Inorganic, neutral. 



Organic contacts will be with other organisms, dead or alive. Such con- 

 tacts are susceptible of almost endless subdivision and classification according 



1 Case, E. C., Carnegie Inst. Wash. Pub. No. 207, p. 115, 1915. 



