26o COOK 



The prompt loss of wool by sheep brought to tropical coun- 

 tries is one of the most striking instances of response to environ- 

 mental conditions, but there are several elements which need to 

 be taken into account in attempting to arrive at a clear under- 

 standing of the nature of the process. The continuous heat 

 and excessive humidity may induce an abnormal condition of 

 the skin and cause the hair to fall out, as often happens in hu- 

 man fever-patients. On the other hand, the failure of the sheep 

 raised in the tropics to produce wool may be due to a lack of 

 sufficiently normal conditions of existence which disturbs the 

 normal heredity and affects first the most highly specialized 

 character of the animal. The loss of wool could be explained 

 in this way as a deterioration or reversion rather than as a new 

 or adaptive character. The domestic sheep is now supposed 

 by Lydekker to be descended from wild types which had a 

 hairy summer coat and produced wool only as cold weather 

 approached. 1 



Many animals and plants require the seasonal vicissitudes of 

 heat and cold as a normal part of the conditions of existence, 

 and refuse to behave normally in tropical regions where wide 

 ranges of temperature do not occur. 2 Indeed, the changes of 

 temperature appear to supply to some of them the same kind of 

 bodily vigor to which diversity of descent contributes. The 

 plants and animals of. tropical regions appear to have rela- 

 tively great rapidity of evolutionary progress, as pointed out 

 by President Jordan, who finds that the tropical fishes are much 

 more highly specialized than those of extratropical waters. 



"The processes of specific change, through natural selection 

 or other causes, if other causes exist, take place most rapidly 

 there and produce most far-reaching modifications." 3 



It has not been shown, however, that natural selection is less 

 acute in the colder regions of the globe ; in fact, the general 

 impression has been that the requirements are the more stringent 

 and exacting. 



'Lydekker, R., 1904. The Field, 104: 654. 



2 Apples, cherries and many other temperate trees and cultivated plants fail 

 to reach productive maturity under consistently tropical conditions, just as the 

 seeds of lettuce may refuse to sprout without alternations of temperature, and 

 the eggs of some mosquitoes refuse to hatch unless they have been frozen. 



3 Jordan, D. S., 1901. Science, N. S., 14: 566. 



