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 
