ANIMAL BREEDING 197 



in Chapter IV; therefore it will not be necessary to 

 consider this factor again. 



Turning now to the question of crowding, we find 

 that as a result of experience it has long been known 

 that such a condition acts adversely on caged animals, 

 although in the case of higher animals little actual ex- 

 perimentation has been directed towards a study of 

 this factor. What literature exists on this topic has 

 recently been reviewed by Allee (1931), and the 

 reader is referred to his book and the excellent mono- 

 graph on the rat by Greenman and Duhring ( 1923) , 

 for the details of experiments made by others. 



At first it is rather difficult to understand why the 

 overcrowding of cages should lead to the ill health 

 of animals except in so far as it limits their freedom 

 of motion, thus preventing a normal amount of exer- 

 cise. Undoubtedly this is an important factor in the 

 effects of overcrowding, but, as will be shown below, 

 it is not the only one. 



The reader may recall that it has been shown that 

 the toxic effects of cytost and the respiratory involve- 

 ment attendant thereto may be induced in cats simply 

 by spraying their cages with an extract of homologous 

 cytost, or by the application of a paste of autolyzed 

 tissue to the paws of the animals. This finding demon- 

 strates among other things that when present in the 

 air surrounding the animals cytost may enter the re- 

 spiratory tract and there set up a congestion. 



Now other things being equal, the extent and 

 severity of such a congestion will be dependent upon 

 the amount of cytost inspired and this in turn must 

 bear some relation to the amount present in the air. On 

 this basis, it follows that as a result of inhaling air con- 



