130 Dynamic Theory. 



however, habit and use have produced a wonderful variety of adapta- 

 tions. The process of acclimation sometimes requires several genera- 

 tions, and in reality the anatomical differentiation must be considerable 

 in many cases in which it is not externally apparent. In Germany the 

 Amphibians come out of their winter sleep when the temperature gets 

 up to freezing point, but in Cuba, where they have become habituated 

 to a normal temperature much higher, they cannot keep awake at a tem- 

 perature of 7 C. and some of them go to sleep at 24 C. In the Phil- 

 ippine Islands snakes succumb to 16 to 18 C. or 61 to 64 F. 



Bayard Taylor, after spending a winter in Lapland, found the tempera- 

 ture of freezing point oppressively warm. It is said that animals in 

 the Arctic Ocean will die if exposed to a temperature of 30 C. ( 86 F.) 

 in the rays of the sun. The change of color which takes place in some 

 animals on the approach of cold weather, appears to be another effect 

 connected with the same causes that produce drowsiness. Brown weas- 

 els and some gray rabbits turn white on the approach of winter. It is 

 easy to see that this is an advantage in the snowy regions of the tem- 

 perate zones, in making the animals less conspicuous, and must there- 

 fore be a cause of selection. But the cause of the modification appears 

 to be the same underfeeding which is due to cold and which, instead of 

 being shared by the various tissues and producing sleep, is concentrated 

 chiefly on the skin, cutting down its function of secreting and conserv- 

 ing the coloring matters. The white color is the best reflector of lumin- 

 ous heat rays, so that an animal having a white coat will get less heat 

 from the sun, which is probably a disadvantage in a cold climate; while 

 the white coat will radiate his own obscure heat as readily as a colored 

 one. The tropical black probably arises also from defective color func- 

 tion at the other extreme, because black being a bad reflector of lumin- 

 ous heal, it must be regarded as an unfavorable color in a hot, sunny 

 climate. 



In one of these extremes it would appear that the tone of the system 

 was unequal to the task of secreting the proper amount of coloring mat- 

 ter, in the other it was unable to get rid of a superabundance of it. 

 Albinos present an example of the defect. They are said to be common 

 among the black races. The bleaching of "the hair in age is another ex- 

 ample of the letting down of the function, by degrees. In the bleach- 

 ing which sometimes follows excessive fright, it is dropped suddenly. 



An anomaly or freak, which at first occurs as an accident of an or- 

 ganism, if repeated often enough through the persistence of the cause, 

 will become habitual to that organism; and if afterward the original 

 cause that brought it about shall disappear the habit of the organism 

 may still be perpetuated. Sooner or later, however, these habits will 

 disappear, and the anatomical peculiarity which may have been caused 



