small shells and put into larger ones they may grow up to be as large as animals of typical size. These dwarfs are, therefore, only a physiological 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 difference 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 characteristic 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 dissimilation exceeds assimilation they dwindle. The large-sized Crepidulæ 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 development which limits growth and that it appears earlier in some species than in others. Since assimilation and dissimilation are chemical processes 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
