CONDITIONS OF CELLULAR IMMORTALITY 69 



which is even more apparent in his long ones, namely, 

 that the figures for duration of life are distinctly round 

 numbers. There is no evidence, for example, that the 

 normal life span of the mouse is 6 years. All who have 

 statistically studied the matter agree upon a much smal- 

 ler figure than this. But, leaving this point aside, it is 

 apparent that there is a parallelism of striking sort be- 

 tween the cephalisation factor and duration of life. In 

 other words, it appears that the manner in which higher 

 vertebrates, at least, are put together in respect of the 

 proportionality of brain and body is markedly associated 

 with the duration of life. It would be a matter of great 

 interest to see whether this correlation between relative 

 brain-weight and the expectation of life holds intra- 

 raciaUy as well as it does inter-racially. The bearing of 

 these results of Friedenthal's upon our results as to the 

 distribution of mortality upon a germ-layer basis, to be 

 discussed in Chapter V infra, is obvious. 



Another possible illustration of the general point 

 now under discussion may be found in some recent work 

 of Robertson and Ray. These authors, in a recent paper, 

 have analyzed the growth curves of relatively long-lived 

 mice as compared with the curves shown by relatively 

 short-lived individuals. In the experiment both groups 

 were subjected to the same kind of experimental treat- 

 ment of various sorts, and the care with which the experi- 

 ments were conducted in respect of control of the 

 environmental factors renders the results highly inter- 

 esting and valuable. The long-Kved animals form a group 

 which grows more rapidly in early life, and at the same 

 time is less variable than the short Kved group. The 

 short-lived animals often grow much more rapidly in 

 later life than the long-lived, but this accretion of tissue 



