254 



But we found also lO increasiiif^ proportionally to L or Z^'^». And 

 this is the same ratio as exists between the mass of the body and 

 tlie muscular force, the metabolism, the rate of conduction of the 

 nerve impulses. 



It has been found cjtologically that with constant relation of 

 nucleus and plasma also the rate of cell division remains con- 

 stant. And already in J895 Alkxandkr Suthkhi.and ^) had shown that 

 the time of incubation of bird species and the time of p;estation of 

 related speckles of mammals increases proportional to P^l^ or V^L; 

 weight and length being those of the full-grown animal's body. 



6 



In general this time is T=it\^P, in which ii is a constant, 

 almost the same for all bird species, but different for every order 

 or family of the Mammalia, which tends to increase with the 

 increase of "nerve complexity, as gauged by size and efliciency of 

 brain". Its amount is in indubitable connection with that of the 

 coefficient of cephalisation a, whi(;h is determined by the hetero- 

 neuric increase of the number of nerve cells; but n certainly 

 increases less greatly and is, in Mammalia, also dependent on other 

 circumstances (as the non-coincidence of the dates of copulation 

 and fecundation). The values n and x are highest in Man, Apes, 

 and the Elephant. The 105 bird species mentioned by Suthkrland 

 differ relatively little inter se in their cephalisation, but in some its 

 influence on the time of incubation can yet be recognized, such in 

 the Owls in comparison with the Gallinae. Thus the time of growth, 

 determined by cell division, to birth appears to be in the same 

 relation to the body weight of the adult animals as the nucleus 

 volume of full-grown homologous nerve cells, which cease dividing 

 at birth. This means equal increase of the number of nerve cells 

 to their separate volume. Again, finished cell divisiori in the brain 

 implying completion of linkage in the nervons integrative machinery, 

 it thereby causes mechanically birth, of mammal as well as bird. 

 In the origination of a heteroneuric species the phylogenetic 

 growth of the brain volume is not uniform, in simple mechanical 

 accordance with the phylogenetic growth of the body, as in the 

 establishment of a larger homoneuric species, but it is stronger in 

 those most compounded parts of the brain, where new chains of cells 

 are superposed upon the preexisting chains, superiorly integrating 



1) Alexander Sutherland, Some Quantitative Laws of Incubation and Gestation. 

 Proceedings of the Royal Society of Victoria. Vol. Vll. (New Series), p. 270- 286. 

 Melbourne 1895. Also in 'llie Origin and Growth of the Morallnstinct, p. 69— 71 and 

 101—102. London 1898. 



