CHAPTER III 



THE EFFECT OF ENVIRONMENTAL SELECTION 

 ON ANIMAL DISTRIBUTION 



The factors which condition animal existence are favorable in 

 varying degrees at different places on the earth's surface. The most 

 favorable environment of all is afforded by the littoral areas of trop- 

 ical seas. Here the salt content is subject to little variation; the tem- 

 perature is nearly constant at about 25°, with an annual variation of 

 only 2.3°; and abundant food supply is brought in from the land. The 

 fauna of these seas is richer in variety of form and color than can be 

 found anywhere else. All the phyla of the animal kingdom are repre- 

 sented; no unfavorable conditions require special adaptations; and 

 rank growth with a high tendency to variation combined with the 

 severe struggle for existence between different animals has produced 

 the greatest amount of differentiation. These optimum physicochemical 

 conditions for animal life may be compared with those afforded to the 

 embryos of birds and mammals, which develop in the brooded egg or 

 in the uterus at a constant and most favorable temperature, in an 

 isotonic environment, and are nourished adequately and without 

 effort. 



Wherever the environmental conditions deviate from such an opti- 

 mum, become disadvantageous, and approach the limiting conditions 

 of either extreme, an impoverishment takes place in the number of 

 major groups represented and the general diversity of the fauna. Many 

 types of organization, and many individual species, are unable to 

 withstand such deviations, or to transform themselves in adaptation 

 to them. Many groups of animals are therefore entirely absent, and 

 others flourish less and tend less toward variation and speciation. Un- 

 der the stress of conditions which urgently demand adaptation, the 

 number of species diminishes. 



Adaptations to similar environmental factors will not, in general, 

 be able to take many different paths. They will thus produce a certain 

 similarity, and will introduce convergent traits in the inhabitants of a 

 given environment even among those of diverse ancestry. Such com- 

 mon characteristics may result from the direct influence of external 

 factors; as for example, the lowering of the temperature results in a 



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