THE PHYSIOLOGY OF REPRODUCTION 



said to have been due to the change in his mode of life, resulting 

 from his migration from Shropshire to London, " where he fed 

 high and drunk plentifully of the best wines." " He died 

 after he had outlived nine princes, in the tenth year of the tenth 

 of them, at the age of one hundred and fifty-two years and 

 nine months." 



As to what factors determine the average duration of life in 

 difEerent species is a problem about which there has been much 

 speculation. Weismann has elaborated a theory which asserts 

 that living matter was originally immortal, mortality first arising 

 in correlation with cellular differentiation. On this view the 

 Protozoa are potentially immortal,^ natural death occurring 

 only among multicellular organisms. The protoplasm of the 

 latter is shown to be of two kinds — ^germplasm, which is capable 

 of propagating itself indefinitely under suitable conditions like 

 the protoplasm of unicellular organisms, and somatoplasm, 

 which composes the rest of the body and is subject to natural 

 death. The life of the somatic cells was at first limited to one 

 generation, but afterwards in the higher Metazoa was extended 

 to many generations, and the life of the organism was lengthened 

 to a corresponding degree. Such a restriction went on hand in 

 hand with a differentiation of the parts of the organism into 

 somatic and reproductive cells, in accordance with the principle 

 of the physiological division of labour, and this process of 

 differentiation was controlled by natural selection. " Death 

 itself," says Weismann,^ " and the longer or shorter duration of 

 life both depend entirely on adaptation. Death is not an 

 essential attribute of Uving matter ; it is neither necessarily 

 associated with reproduction, nor a necessary consequence of 

 it." According to this theory, therefore, the phenomena of 

 senescence and death, as exhibited by all the cells of the body 

 with the exception of the germ cells, are secondary properties 

 which have been preserved in multicellular organisms by natural 

 selection, because they are of direct advantage in the propaga- 

 tion of the species. An indefinite prolongation of the life of the 

 organism after the age of reproduction had been passed would 



^ This question, about which there has been much controversy, is referred 

 to in Chapter VI. (pp. 212-14). 



^ Weismann, " Life and Death," Essays, vol. i., 2nd Edition, Oxford, 1891. 



