86 SENESCENCE AND REJUVENESCENCE 



animals, and the capacity of an individual for growth and develop- 

 ment must be regarded as to some extent a criterion of its youth or 

 age. If we can induce an animal to pass through an indefinite 

 number of agamic generations, each of which shows the same vigor 

 and the same cycle of growth and development, we must conclude, 

 either that senescence does not occur in such cases, or else that 

 there is a periodic rejuvenescence associated in some way with the 

 reproductive process or other processes, and we may use the sus- 

 ceptibility methods to determine which of these two alternatives 

 is correct. In at least many organisms, probably in all, if the 

 nutritive and other conditions are controlled with sufficient care, 

 the percentage increment of growth decreases with advancing 

 age and serves as a more or less exact indication of physiological 

 condition, though subject to periodic or irregular variation. In 

 those forms which attain or approach a more or less definite limit 

 of size, size itself under the normal or usual conditions of existence 

 may serve as a criterion of age, since the size of the organism indi- 

 cates approximately its position in the life cycle. 



The morphological characters, whether those of the cells or of 

 the organism as a whole, may serve as an indication of the youth or 

 age of the individual, but it must be remembered that senescence 

 and rejuvenescence are primarily physiological rather than morpho- 

 logical changes, and that morphological characters are available 

 as criteria only so far as we have learned by experience that cer- 

 tain of them are characteristic of organisms which we can distin- 

 guish by other means as physiologically young or old. In man and 

 the higher animals the morphological differences between youth and 

 age are clearly evident, but for many of the lower forms this is not 

 the case, although sufficiently minute anatomical or histological 

 investigation would probably disclose some characteristic differ- 

 ences. If these various criteria of youth and age are all valid, we 

 should find that, so far as they can be applied to any particular case, 

 they lead to essentially the same conclusion as regards that case. 

 As a matter of fact, they are very generally in agreement, but 

 there are various cases in which one or another of these criteria 

 leads to conclusions different from the others. Some of these 

 cases will be considered in later chapters. 



