LIFE AND DEATH. 145 



accurate conception of what is meant by a corpse, proves the neces- 

 sity for arriving 1 at a scientific idea as to the meaning- of the latter. 

 There is no death without a corpse whether the latter be small 

 or large, whole or in pieces. 



If we compare the bodies of the higher Metazoa with those of 

 the lower, we see at once that not only has the structure of the 

 body increased in size and complexity as far as the soma is con- 

 cerned, but we also see that another factor has been introduced, 

 which exercises a most important influence in lengthening the 

 duration of life. This is the replacement of cells by multipli- 

 cation. Somatic cells have acquired (at any rate in most tissues) 

 the power of multiplying, after the body is completely developed 

 from the reproductive cells. The cells which have undergone 

 histological differentiation can increase by fission, and thus supply 

 the place of those which are being continually destroyed in the 

 course of metabolism. The difference between the higher and 

 lower Metazoa in this respect lies in the fact that there is only 

 one generation of somatic cells in the latter, and these are used 

 up in the process of metabolism at almost the same time that the 

 reproductive cells are extruded, while among the former there are 

 successive generations of somatic cells. I have elsewhere en- 

 deavoured to render the duration of life in the animal kingdom 

 intelligible by the application of this principle, and have attempted 

 to show that its varying duration is determined in different species 

 by the varying number of somatic cell-generations 1 . Of course, 

 the varying duration of each cell-generation materially influences 

 the total length of life, and experience teaches us that the duration 

 of cell-generations varies, not only in the lowest Metazoa as com- 

 pared with the highest, but even in the various kinds of cells 

 in one and the same species of animal. 



We must, for the present, leave unanswered the question upon 

 what changes in the physical constitution of protoplasm does 

 the variation in the capacity for cell-duration depend ; and what 

 are the causes which determine the greater or smaller number of 

 cell-generations. I mention this obvious difficulty because it is 

 the custom to meet every attempt to search deeper into the com- 

 mo.n phenomena of life with the reproach that so much is still 

 left unexplained. If we must wait for the explanation of these 



1 . See the first essay on ' The Duration of Life.' 

 L 



