ORGANIC RESPONSE 255 



such as the shore crabs, the tree-frogs, and the 

 flat-fishes, the color change is protective and 

 brought about under the direct influence of the 

 environment. Another striking effect of tempera- 

 ture is that produced by cold upon hairy animals. 

 The small shaggy ponies of Shetland and Iceland, 

 while in their native land, are provided not only 

 with the ordinary hair characteristic of horses every- 

 where, but also with a dense woolly fur underneath, 

 which serves to keep them warm in very cold weather. 

 When brought into a warmer climate, however, one 

 year suffices to shed this dense coat, and thereafter 

 the hair is no different from that of other horses. 



Immunity. One of the most striking examples of 

 individual adaptation is found in the manner in which 

 the higher organisms (or at least the warm-blooded 

 ones) react against infectious diseases. An organism 

 that is non-susceptible to a specific microbic disease 

 is said to be immune to that disease. This immunity 

 may be " natural " or it may be " acquired." It is 

 probable that all natural immunity has been acquired 

 during the lifetime of the race, through the auto- 

 matic elimination of the non-immune individuals. 

 Artificial immunity, on the other hand, is the individ- 

 ual affair of the organism. In pathology, two kinds 

 are recognized : active immunity in which the tissues 

 of the body react directly against the toxins of the 

 invading bacteria, and passive immunity, which is 

 conferred upon the individual by the injections of 

 blood-serum from another actively immune animal. 



