DURATION OF LIFE. 133 



SECTION XIX. 



DUKATION OF LIFE. 



There is the most extraordinary inequality in the ave- 

 rage duration of the life of different kinds of animals and 

 plants. While some grow and reproduce themselves and 

 die in a short summer, nay, in a day, others seem to defy 

 the influence of time. 1 



Who has thus apportioned the life of all organized 

 beings ? To answer this question, let us first look at the 

 facts of the case. In the first place, there is no conform- 

 ity between the duration of life and either the size or 

 structure or habitat of animals ; next, the system in which 

 the changes occurring during any period are regulated 

 differs in almost every species, there being only a slight 

 degree of uniformity between the representatives of dif- 

 ferent classes, within certain limits. 



In most Fishes and the Eeptiles proper, for instance, 

 the growth is very gradual and uniform, and their de- 

 velopment continues through life, so much so that their 

 size is continually increasing with age. 



In others, the Birds, for instance, the growth is rapid 

 during the first period of their life, until they have 

 acquired their full size, and then follows a period of 

 equilibrium, which lasts for a longer or shorter period in 

 different species. 



In others again, which also acquire within certain limits 

 a definite size, the Mammalia, for instance, the growth is 

 slower in early life, and maturity is attained, as in man, 



1 SCHUBLER, (GusT.,) Beobachtun- und Pflanzenreich, Tubingen, 1831, 

 gen iiber jiihrliche periodisch wieder- Svo. QUETELET, (A.,) Pkciiomeues 

 kuhrendc Ersclieiuuugen im TLicr- p&riodiques, Ac. Brux. 



