REPRODUCTION IN UNICELLULAR ORGANISMS 263 



differentiation in itself excludes the possibility of unlimited length of 

 life and multiplication. Through this alone, therefore, the exhaustion 

 of the body and an ultimate death may be explicable from internal 

 causes. But the deeper cause remains what I have already indicated, 

 for it is obvious that if the continued life, that is, the immortality of 

 the soma, were necessary to the preservation of the species it would 

 have survived through natural selection ; that is to say, had it been 

 so, then histological differentiations incompatible with immortality 

 would not have made their appearance ; they would always have been 

 eliminated on their way to development, since only that which is 

 adapted to its end survives. Only if the immortality of the soma 

 were indifferent for the species could the soma have become so highly 

 organized that it became subject to death. 



Thus the old song of the transitoriness of life does not apply to 

 all the forms of life : natural death is a phenomenon which made its 

 appearance comparatively late in the development of the organic 

 world, a phenomenon which, up to a certain point, we can quite well 

 understand from the standpoint of purposefulness. 



It would take me too far from the goal towards which we are at 

 present making if I were now to attempt to show, in connexion with 

 natural death, that the durability of the soma, or what we usually 

 call the normal duration of life, is also exactly regulated by natural 

 selection, sc that each species possesses exactly that duration of life 

 which is most favourable to it, according to its physical constitution, 

 its physiological capacity, and the conditions of life to which it has 

 to adapt itself. But, interesting as this subject is, I must not digress 

 further, but return to our proper subject of study, namely, reproduction 

 in its relation to inheritance. 



We digressed from this study after having seen that all, even 

 the most complex, multicellular plants and animals, in which the 

 differentiation of the cells into a number of cell-groups with the 

 most diverse functions has attained the highest degree of complexity, 

 are able to produce special cells, the germ-cells, which have the 

 power of reproducing from themselves another organism of the 

 same species, and with the same complex structure. It might be 

 thought that such cells must necessarily be very complex in their 

 own structure, but in most cases nothing of the kind is to be seen, 

 and the germ-cells often appear simpler in organization than many 

 of the tissue-cells, such as the glandular-cells ; and where there is an 

 unusual size or complexity of structure in the germ-cell it usually 



' See Weismann, Ueber die Bauer des Lelens, Jena, 1882. Translated in Essays on 

 Heredity. 



