42 EX VIRIBUS VIVIMUS. 



period of natural death comes to differ in various 

 species by heredity. We have seen how it is pos- 

 sible for a limit to be inherited ; but how does the 

 period so limited come to be an hereditary quantity 

 characterizing species ? How is it that it varies in 

 animals which commence life and carry it on under 

 very much the same conditions ? The specific acci- 

 dents, actions, wear and tear to which different 

 species are severally subjected are not sufficient alone 

 to account for the fixity of the period, though their 

 influence is important. There is something addi- 

 tional, some more direct cause than these, and we 

 must look for it in the quantitative limitation of the 

 germinal matter itself, varying in species. If it were 

 not so, how can we account for the fact that a cow 

 and a sheep,^ which start from ova so exactly iden- 

 tical in form and size, composed probably of equal 

 amounts of germinal matter or protoplasm, subject 

 as they develope to the same external influences, 

 living perhaps side by side in the same field, yet 

 differ in their inherited term of hfe, which appears 

 to be, as nearly as can be guessed, about twenty 

 years for the larger and twelve for the smaller rumi- 

 nant } We have seen that the expenditure increases 

 during growth more rapidly than the bulk, more 

 rapidly a fortiori than the accumulation of germinal 

 matter, which we saw did not increase even as rapidly 



' This illustration and the aiugmeiit are borrowed from Mr. Herbert 

 Spencer, who discusses the question of size. 



