200 EVOLUTION, GENETICS, AND EUGENICS 



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 

 m an exceedingly unmature state and looking like tiny pink grubs. 

 They crawl under their own power, by means of a swimming-like 

 motion, through the hairs of the mother's abdomen, till they reach the 

 pouch. This they enter unaided and each tiny larva finds for itself 

 a slender tubular teat, which it swallows and holds in place by a 

 specially adapted hold-fast mouth. The young remains attached 

 fixedly to this teat for some weeks, feeding almost constantly on milk. 

 After a long interval the teat is released, the mouth metamorphoses 

 into the adult form and the young feeds only at intervals, as do the 

 young of other mammals. This complex of adaptive structures and 

 instincts is among the most remarkable in the annals of biology. 



