282 AN INTRODUCTION TO ZOOLOGY 



and by interbreeding the individuals that showed these traits the 

 ancon race of sheep was established (260). The most famous 

 discontinuous variations known in the plant kingdom are the 

 distinct species of evening primroses, which have originated in 

 recent years from the common primrose, and have been so well 

 described by the Dutch naturalist de Vries (272). 



(4) Adaptations of Animals 



The adaptations of animals to their environment is so usual as 

 to pass unnoticed. Each of the types we have studied is fitted 

 for life in its particular habitat, and in most cases in no other, a 

 fact which, as we have seen, accounts largely for the geographical 

 distribution of organisms. Animals not only are adjusted to their 

 surroundings, but possess structures which serve them in obtain- 

 ing food, in defending themselves and their young, and in main- 

 taining supremacy among individuals of their kind. The cray- 

 fish is adapted to its environment by the possession of gills for 

 breathing, eyes for detecting moving objects in the water, a hard 

 exoskeleton to protect it from various enemies, pinchers for offense 

 and defense, and a positive thigmotropic reaction which causes 

 it to seek a dark spot out of range of marauders. 



Insects are especially favorable for illustrating adaptations. 

 The following structures of the honeybee are excellent examples 

 of adaptations: the hairs and legs for gathering pollen, the ali- 

 mentary canal for obtaining and storing nectar, the mandibles 

 for molding wax, the sting for self-defense, for the defense of the 

 hive, and, in the case of the queen, for use in determining her 

 supremacy. The origin of adaptations is a moot question among 

 biologists. They are variations which have been inherited, and 

 have become more pronounced in the course of countless genera- 

 tions. 



