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THE AMEBIC AX NATURALIST 



[Vol.XLVI 



shown that the period of gestation depends upon the state 

 of the female. If the pregnant female is not suckling 

 young, parturition with but rare exceptions takes place on 

 the twentieth day after copulation. If she is suckling on 

 the other hand, the period varies with the number of 

 young suckled. Thus, for example, if the mother be 

 suckling live during gestation she may be expected to go 

 about twenty-five days ; if ten thirty days. 



Parturition, which terminates the period of gestation is 

 normally of brief duration, even in case a large litter is 

 born. But this is not invariable, for I have observed 

 cases in which labor was prolonged, and some in which 

 unaided birth was impossible. 



We arc inclined to believe that in mankind the difficulty 

 of giving birth, which not infrequently results in the death 

 of the mother, is due to the artificialities of civilized life. 

 But here we find the same stern fact emphasized in a type 

 remotely removed from any such influence. 



IV. Rearing of the Young 

 The most hazardous time in the life of a mouse is the 

 first few days of its existence. Born helpless and naked 

 it is dependent upon the mother not alone for nourish- 

 ment but for warmth as well. Some mothers at this time 

 are most solicitous for their young, building elaborate 

 nests for them and giving such care to the young as to 

 tide them over this early period. Others there are that 

 not only withhold the requisite care, but which at this 

 time prove the most serious menace. Some of these 

 bundle their young away in the nesting to die; while 

 others in bad conditions openly destroy them. This sin- 

 gular and, so far as I am aware, unexplained phenomenon 

 of destroying the offspring, is carried to a high pitch in 

 the case of mice. Under unfavorable conditions of tem- 

 perature, nesting and the like, I have seen three or four 

 litters destroyed in succession. Miller (7) in a study of 

 the brown rat shows that this destructiveness in the rat 

 even a greater extent than in the case of the 



