XIII. REPRODUCTION. 



THE principles and problems of Physiology that have been already pre- 

 sented in this volume, comprising nutrition and the functions of the muscular 

 and the nervous systems, have reference to the individual man or woman. 

 Through the normal activity of those functions and their appropriate co- 

 ordination the individual lives his daily life or performs his daily tasks as an 

 independent organism. But man is something more than an independent 

 organism ; he is an integral part of a race, and as such he has the instincts of 

 racial continuance. The continuance of the race is assured only by the pro- 

 duction of new individuals, and the strength of the human reproductive 

 instinct is indicated in some measure by the large proportion of energy that is 

 expended by woman in the bearing of children and by both sexes in the nur- 

 ture and education of the young. The function of reproduction is not limited 

 to the daily life and well-being of independent organisms. It has a deeper 

 significance than that. Its essence lies in the fact that it has reference to the 

 species or race. Many of its problems are, therefore, broad ones ; they in- 

 clude not only the immediate details of individual reproduction, but larger 

 ones relative to the nature and significance of reproduction and of sex, and to 

 heredity. In the following discussion some of these broader applications of 

 the facts presented will be indicated. 



A. REPRODUCTION IN GENERAL. 



In all forms of organic reproduction the essential act is the separation from 

 the body of an individual, called the parent, of a portion of its own material 

 living substance, which under suitable conditions is able to grow into an inde- 

 pendent adult organism. 



Among living beings two methods of reproduction are recognized, the 

 asexual and the sexual methods. Both are widespread among animals and 

 plants, but the asexual method is the more primitive of the two and is rela- 

 tively more frequent in low organisms. The sexual method, the only one 

 present in the production of new individuals among the higher animals, has 

 evidently been acquired gradually, and has probably been developed from the 

 asexual method. 



Asexual Reproduction. Asexual reproduction, or agamogenesis, is the 

 chief method of reproduction among unicellular plants and animals, and 

 throughout the plants and in the lower multicellular animals it is important. 

 Among various species it takes various forms, known as fission or division, 

 gemmation or budding, endogenous cell-formation or spore-formation or multi- 



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