724 THE PHYSIOLOGY OF REPRODUCTION 



As to what factors determine the average duration of life in 

 different 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, 1 natural death occurring only among multi- 

 cellular 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 living 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 be of 

 no value or utility to the race, but rather a disadvantage, since it 

 would tend to retard the evolution of more perfectly adapted forms 

 of life. Furthermore, according to Weismann, longevity, although 

 depending ultimately upon the physiological properties of the cells, 

 is capable of adaptation to the conditions of existence, and consequently 

 is influenced by natural selection just in the same way as other 

 specific characters are. 



Perhaps the most cogent criticism of Weismann's doctrine of 

 i in mortality is that of Verworn, who writes as follows: "The 

 conception of living substance as immortal will be accepted by 

 scarcely anyone who bears in mind the characteristic peculiarity 

 of living substance, viz., that it continually decomposes, or, in other 

 words, dies. There is no living substance that, so long as it is living 



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

 to in Chapter VI. (pp. 220-224). 



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



