8 ECOLOGICAL FOUNDATIONS OF ZOOGEOGRAPHY 



unique events it studies processes which are largely repeated like 

 chemical reactions or physical experiments. It is true that the posi- 

 tion occupied by an animal is conditioned by its heritage, but there 

 is also an influence of the environment upon the animal, by natural 

 selection if by no other means, and the modifications thus acquired 

 are frequently of an adaptive nature, i.e., they make it easier for the 

 animal to live in its environment. 



It is accordingly one of the most important problems of ecological 

 zoogeography to investigate the adaptations of animals to their en- 

 vironments. An animal may become adapted to the conditions of its 

 existence by somatic and by genetic processes. The most frequent and 

 most important form of adaptation is somatic, or functional adapta- 

 tion. Whether an organ is passive or directly active, its capacity often 

 is increased by use, owing to the wonderful property of living matter 

 to react adaptively. Such processes are necessarily repeated when the 

 same conditions are supplied. The enlargement of the mammalian 

 heart with increased bodily activity, the thickening of the shells of 

 mollusks under the influence of wave action, and the enlargement of 

 the kidneys in fresh-water animals with increased necessity for excre- 

 tion, are examples. Other somatic changes which appear with equal 

 certainty in consequence of environmental stimuli may be of indif- 

 ferent value to the organism. Examples are the reduction in size of 

 marine animals with decrease in the salinity of the water, and many 

 changes in coloration induced by increased or decreased temperature. 

 Such directly conditioned changes may accidentally prove to be of 

 value to the animal, and they may then be designated as coincident 

 adaptations. In this way the colors of the most diverse animals lose 

 their brilliance under the dry heat of the desert, become pale and 

 dull, and thereby cause the animals to resemble the desert floor. The 

 darkened coloration exhibited by many Lepidoptera when subjected 

 to cold during pupation may favor the warming of the body of the 

 adult in the northern species. Again, in the small crustacean of saline 

 waters, Artemia salina, the relative size of the supporting surfaces 

 is increased with decreasing density (i.e., supporting ability) of the 

 salt solution. 



Germinal changes also give rise to new characters in animals. 

 These mutations are for the most part of no importance to the effi- 

 ciency of the animal; but they may sometimes be of value, and then 

 they may be preserved by selection and become more and more widely 

 distributed. 



Special adaptations, physiological or structural, are the more neces- 

 sary to animals the further the conditions of their environment depart 



