CHAPTER XXXV 
FERTILITY IN ANIMALS 
Among domestic animals fertility is of direct economic importance. 
Problems associated with it have been investigated from many different 
angles, even from the standpoint of inheritance. Unfortunately, how- 
ever, with respect to this latter feature of the question, not many inves- 
tigations have been carried out with higher animals. It is necessary, 
therefore, to seek for the principles disclosed by investigations with the 
lower forms of life, and to determine to what extent they may be applied 
to higher animals. 
Factors Influencing Fertility—The factors which affect fertility are 
extremely numerous and varied. In considering the problems of in- 
heritance connected with it, it is, therefore, necessary to make the inevi- 
table scientific distinctions as to kinds of influences which may affect it 
and as to the different meanings which the term itself may have. In 
common parlance the term fertility signifies ability to produce active, 
living young. In higher animals in general fertility is measured by 
the reproductive capacity of pairs of individuals. Fecundity is the 
term used to designate the potential reproductive capacity of indi- 
viduals. It is measured by the ability of the individual to form mature 
ova or spermatozoa. Fecundity can be measured accurately and di- 
rectly only in special cases such as in birds; in mammals only fertility 
can be determined. 
Several physiological factors must be considered in a treatment of 
fertility in animals. Of these only a few can be mentioned here. For 
a more extended treatment, the student should consult treatises on the 
physiology of reproduction, of which that of Marshall is especially 
valuable. 
Among influences which lead to sterility or decrease in fertility are 
those of domestication. Here the effect depends largely upon the idio- 
_ syncracies of the particular wild species which has been domesticated. 
It has been suggested that food might in some cases be the determining 
factor, the supposition being that the animal under captivity may not 
obtain the variety and character of food necessary to maintain a healthy 
condition of the reproductive tracts. By no means, however, is the 
sterility of wild animals in captivity visibly correlated with changes 
in their mode of life, for often the most surprising variations occur in 
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