I40 LIFE AND DEATH, [III. 



the most tortuous and winding route, so the structure of the 

 original organism has undergone manifold changes during its 

 terrestrial life. And just as the traveller at first doubts whether 

 he will ever get beyond the immediate neighbourhood of his 

 starting-point, and yet after some years finds himself very far 

 removed from it — so the insignificant changes which distinguish 

 the first set of generations of an organism lead on through 

 innumerable other sets, to forms which seem totally different 

 from the first, but which have descended from them by the 

 most gradual transition. All this is so obvious that there is 

 hardly any need of a metaphor to explain it, and yet it is 

 frequently misunderstood, as shown by the assertion that 

 natural selection can create nothing new : the fact being that 

 it so adds up and combines the insignificant small deviations 

 presented by natural variation, that it is continually producing 

 something new. 



If we consider the introduction of natural death in connection 

 with the foregoing statements, we may imagine the process 

 as taking place in such a way that, — with the differentiation of 

 Heteroplastids from Homoplastids, and the appearance of 

 division of labour among the homogeneous cell-colonies, — 

 natural selection not only operated upon the physiological 

 peculiarities of feeding, moving, feeling, or reproduction, but 

 also upon the duration of the life of single cells. At this 

 developmental stage there would, at any rate, be no further 

 necessity for maintaining the power of limitless duration. The 

 somatic cells might therefore assume a constitution which 

 excluded the possibility of unending life, provided only that 

 such a constitution was advantageous for them. 



It may be objected that cells of which the ancestors possessed 

 the power of living for ever, could not become potentially 

 mortal (that is subject to death from internal causes) either 

 suddenly or gradually, for such a change would contradict the 

 supposition which attributes immortality to their ancestors and 

 to the products of their division. This argument is valid, but 

 it only applies so long as the descendants retain the original 

 constitution. But as soon as the two products of the fission of 

 a potentially immortal cell acquire different constitutions by 

 unequal fission, another possibility arises. Now it is con- 

 ceivable that one of the products of fission might preserve 



