AN ESSA V ON LONGEVITY. 45 



the rationale of that connection between time of 

 growth, time of gestation, and potential longevity, 

 which has been pointed out,' though we can see 

 no good reason why the number 5 or any other 

 number should express the ratio for a whole class of 

 animals. 



6. Elements of the Life Period. 



It is obvious that by increasing the duration of any 

 one part of the lifetime of an organism, the sum total 

 or whole may be lengthened, and it may be useful, 

 therefore, to consider what are the phenomena which 

 most clearly involve periods of time for their manifes- 

 tation in organisms. 



They are, ist, the period of evolution (including in 

 this term both growth and development) ; 2nd, the 

 period through which reproductive activity is spread ; 

 3rd, the period of dissolution or decay. Any condi- 

 tion of existence which necessitates the lengthening 

 or shortening of one of these periods for the require- 

 ments of the organism directly affects longevity. In 

 the period of evolution are included the period of 

 embryonic development, of foetal life and incubation, 

 the larval period, which many animals exhibit, the 

 period of growth as indicated by osseous completion 

 or similar perfection, or the assumption of the mature 



' Both by riourens and Buffon, as well as the earliest writers on 

 longevity. 



