196 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



Some very common adaptations may belong to several of these 

 categories at once. Thus the sharp teeth and hooked claws of car- 

 nivorous mammals serve equally well for food-securing, for self- 

 defense, for defense of young, and for rivalry. Similarly, the horns 

 of deer and other ungulates are equally adapted for self-defense, 

 defense of young, and rivalry. 



There can be no especial advantage, in this connection, in present- 

 ing a detailed review of adaptations of the sorts given in the foregoing 

 classification; therefore we shall confine our efforts to a description 

 of a few typical adaptations about which the greatest controversy 

 has raged. 



SOME SPECIAL ADAPTATIONS 



The electric organ of the torpedo, a widely distributed elasmo- 

 branch fish, consists of a sort of honeycomb-like structure on each side 

 of the head. This structure acts as a storage battery and is capable 

 of storing up electricity of considerable voltage. The animal is 

 capable of giving a very distinct shock to an attacker and can thus 

 defend itself quite effectively. There is also an electric eel, native to 

 the waters of Paraguay and Brazil, that is able to give severe shocks to 

 bathers or to horses driven through the streams. A type of catfish 

 native to the river Nile has a similar electric equipment. In all of 

 these cases the storage battery is made up of modified voluntary 

 muscles and is of considerable size. 



The mammary glands of mammals are skin glands usually with 

 well-defined ducts leading to the surface and terminating in teats. 

 These glands are quite voluminous and serve admirably the purpose of 

 feeding new-born young until the latter are able to use the more varied 

 food normal to the adult. In the lowest mammals, the monotremes 

 or egg-laying mammals, these glands are relatively poorly developed 

 and diffuse; also they are known to be developed through a regional 

 specialization of sweat glands. In the true mammals or Eutheria the 

 glands are modified sebaceous or oil glands and may be seen to develop 

 from the same embryonic rudiments as the latter. 



The marsupial pouch of the kangaroo and its allies is a pocket- 

 like fold of the integument, folded forward or backward over the region 

 of the abdomen in which are located the mammary glands. This 

 pouch is used as a shelter for the tiny immature larval foetuses. 

 Hartmann has recently described a very striking piece of behavior in 

 connection with the birth of young opossums. The young are born 



