^^6 GROWTH AND THE AGING PROCESS I4 



ages 3 to 13. In other mammalian species the end of the weaning period is very 

 close to the onset of sexual maturity, and they may be said to have little or no 

 juvenile period (Fig. 2). 



Data for a large number of species of mammals and birds (Brody, 1945) indicate 

 that in each case the duration of life is directly proportional to the length of time 

 to reach mature size and inversely proportional to speed of maturation (Fig. 3). 



There are specific instances where the rate of "aging", as measured by change 

 in a particular characteristic of a tissue or an organ, can be shown to run parallel 

 with the rate oi^ decrease in the growth curve. An interesting example of this phenome- 

 non is seen in decline in accomodation of the eye (Fig. 4). 



The growth characteristics of cold-blooded vertebrates are different from those 

 of birds and mammals. They are examples of indeterminate growth (Backman, 

 1938). In fishes growth appears to continue throughout life, only slowing down 

 as the animal becomes older (Fig. 5). By means of the scale method of age deter- 

 mination in fishes of temperate waters, it has been possible to show also that the 

 fertility of female fish, as measured by rate of egg production, actually increases 

 with age. This is shown in data from Raitt (1932) on haddocks of the North Sea, 

 as follows: 



Number of eggs Tear of Ufe 



31,000 second 



100,000 third 



159,000 fourth 



224,000 fifth 



278,000 sixth 



Bourliere (1953) has obtained evidence for a similar continuing growth in snakes 

 and a continuing increase in fertility. Thus the evidence is that in cold-blooded 

 vertebrates, where growth is indeterminate, the decrease or loss of fertility seen 

 in warm-blooded females is not encountered. This is not to say that some evidences 

 of senile decline cannot be found in cold-blooded forms, but it does indicate that 

 where no marked termination of growth occurs, senile decline is a much less clear- 

 cut process. 



There is, then, a definite chronological relationship between growth and senes- 

 cence. Thus, the first task of our three tasks of comparison has been accomplished. 



Is senescence compatible with growth? Robertson (1923), in a penetrating 

 work, stressed the nature of aging as an antithesis of growth. The upholding of 

 this view, of course, must presuppose to some extent that we understand what we 

 mean by senescence. We must realize that the precipitating cause of death in old 

 organisms is not an actual part of the process. Thus the rupture of a bloodvessel 

 in a vital organ such as the heart or brain may terminate the life of the organism. 

 '"Senescence" was involved here perhaps in the subtle tissue changes which led to 

 the greater fragility of the wall. Yet even this change may be more of a pathological 

 nature and not a part of a "natural" senescence any more than is the invasion of 

 pneumococci in an elderly individual who has been hospitalized with a fractured 



