THE SIZE OF ORGANISMS i79 



small shells and ])ut into larger ones they may grow up to be as large as 

 animals of typical size. These dwarfs are, therefore, only a physio- 

 logical variety, produced by environmental conditions. 



What are the causes of such differences in size of animals of the 

 same species? What explanation can be offered for the enormous dif- 

 ference in size between an elephant and a mouse ? What are the factors 

 generally involved in determining size? 



1. There is plainly an inherited factor in all specific differences of 

 this kind. Every species of animal and plant has a more or less char- 

 acteristic body size which may be said to constitute the norm of that 

 species. This norm may be altered to a certain extent by environmental 

 conditions, but such possible modifications are relatively slight; no 

 amount of environmental influence could ever make a mouse grow to 

 the size of an elephant. The limits of body size of a race or species are 

 as certainly inherited as are any other characteristics; their causes, 

 whatever they may be, are intrinsic in the constitution of the germinal 

 protoplasm. 



What is the nature of this inherited factor which determines the 

 possible size of organisms? Undoubtedly it is found in the power of 

 growth as contrasted with limitations to growth, with the rate and 

 duration of assimilation as contrasted with dissimilation. Increase in 

 size may be due to mere imbibition of water, or to an actual increase in 

 the quantity of protoplasm, and secondarily of formed products, in the 

 body. In this discussion the latter process alone will be termed growth. 

 As long as assimilation exceeds dissimilation organisms grow, when the 

 one balances the other they remain unchanged in size, when dissimila- 

 tion exceeds assimilation they dwindle. The large-sized Crepidul(B 

 continue to grow for a much longer time than the small-sized ones. A 

 mouse achieves its full growth after 60 days and may live approximately 

 60 months; an elephant continues to grow for about 24 years and may 

 live approximately 150 years. 



What it is which keeps up this process of growth so much longer 

 in one species than in another we do not know — and as so often happens, 

 it is precisely this which we most desire to know, for length of life as 

 well as size of body depends primarily upon the rate and duration of 

 assimilation. It may be that there is some peculiar secretion or enzyme 

 which stimulates growth and varying quantities of which cause one 

 species to continue to live and grow for a much longer time than another 

 species ; it may be that some substance is formed in the course of devel- 

 opment which limits growth and that it appears earlier in some species 

 than in others. Since assimilation and dissimilation are chemical pro- 

 cesses it is very probable that the factors which determine rate and 

 duration of growth, and consequently body size and length of life, are 

 of a chemical nature. This is a subject upon which there has been 



