CHAPTER XI. REPRODUCTION. 



1. Maintenance of the Species. We have now really 

 completed our survey of the individual animal's mechanism. 

 But no animal that was merely complete in itself would be 

 long sanctioned by nature. For an animal species to 

 survive, there must evidently, also, be proper provision for 

 the production of young and the preservation of the species 

 as well as of the individual. Hence in an animal's physi- 

 ology and psychology we meet with a vast amount of un- 

 selfish provision, and its structure and happiness are more 

 essentially dependent on the good of its kind than on its 

 narrow personal advantage. The mammalia probably owe 

 their present dominant position in the animal kingdom to 

 the exceptional sacrifices made by them for their young. 

 Instead of laying eggs and abandoning them before or soon 

 after hatching, the females retain the eggs within their 

 bodies until the development of the young is complete, and 

 thereafter associate with them for the purposes of nourish- 

 ment, protection, and education. In the matter of the 

 tail, for instance, already noted, the individual rabbit 

 incurs the disadvantage of conspicuousness from the rear, in 

 order to further the safety of the young. 



2. Essential and Accessory Organs of Reproduction. 



The essence of reproduction of the kind we know among 

 the higher animals is the separation of a single nucleated 

 cell from each of two animals, and the union of these two 

 cells into one, which, by rapid growth, division, and 

 differentiation of the cells formed by its division, develops 

 into a new individual. The essential organ of reproduction 

 is one in which special cells for this purpose are developed : 

 such an_ogaiL js_alle4^a_gonad. All other parts of the 

 reproductive system are merely accessory to this serving 

 either as a duct to transmit the reproductive cells to the 

 exterior (gonoduct), or to ensure (directly or indirectly) the 



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