SOME GENERAL CONCLUSIONS 461 



degree of physiological integration, the more continuous the prog- 

 ress of senescence and the less frequently does vegetative agamic 

 reproduction occur. In the plants and lower animals conditions 

 which decrease physiological dominance and integration bring 

 about reproduction of one kind or another. Senescence is itself 

 such a condition, and in many organisms senescence may result 

 automatically in the physiological isolation of parts, or the disinte- 

 gration of the individual into fragments or cells, and so in repro- 

 duction. 



Senescence is a characteristic and necessary feature of life and 

 occurs in all organisms, but in many of the lower forms it may be 

 more or less completely balanced by rejuvenescence in connection 

 with reproduction or other regressive changes, so that there is 

 little or no progressive senescence from one generation to another, 

 or in the case of colonial forms, such as multiaxial plants, of the 

 colony as a whole. Life in such cases consists of brief alternating 

 periods of progression and regression, of senescence and rejuvenes- 

 cence, which in some cases apparently balance each other for an 

 indefinite period, while in other cases a slow progressive senescence 

 may occur, extending through many generations. 



Death is the inevitable end of the process of senescence when 

 regression and rejuvenescence do not occur. In the lower forms, 

 where agamic reproduction is frequent, or where other conditions, 

 such as starvation, bring about regression periodically or occasion- 

 ally, death does not necessarily occur. But in the higher forms, 

 where progression and senescence are more nearly continuous, the 

 life of the individual usually ends in death, though even in these 

 forms some degree of rejuvenescence may occur. 



If these conclusions are correct, agamic and gametic reproduc- 

 tion are fundamentally similar processes, except for the fact that in 

 gametic reproduction specialization of the reproductive cells has 

 proceeded so far that the peculiar conditions associated with ferti- 

 lization are necessary for the initiation of the process of regression 

 and rejuvenescence. And if we accept this theory of reproduction, 

 the Weismannian conception of germ plasm as a self-perpetuating 

 entity, independent of other parts of the organism except as regards 

 nutrition — ^in short, a sort of parasite upon the body — becomes not 



