356 THE EVOLUTION THEORY 



sarily involves that they should wear themselves away in the 

 performance of their functions, and so become subject to death, or at 

 least that they should undergo such changes that they are no longer 

 capable of functioning properly, so that thus the organism as a whole 

 loses the power of life. 



There can be no doubt whatever that death is virtually implied 

 in the very constitution of a multicellular organism, and is thus, 

 so to speak, a foreseen occurrence, the inevitable end of a development 

 which begins with the egg-cell and reaches its highest point with the 

 liberation of the germ-cells, that is, with reproduction, and then 

 enters on a longer or shorter period of decadence, leading to the 

 natural death of the individual. 



It is only by straining the analogy that this course of develop- 

 ment can be compared with the origin and transformation or extinc- 

 tion of species. Xot even tlie entirely external analogy of the 

 blossoming from a small beginning and the subsequent decay is 

 always correct ; for in the fresh- water snails of Steinheim. at any rate, 

 almost the whole of the members of the species underwent a 

 transformation at a particular time, and became a new species, 

 which was after a long time re transformed without any appreciable 

 decrease in the number of individuals being observable. To speak 

 of a ' senile stage ' of the species, of a stiffening of its form, of an 

 incapacity for further transformation, is to indulge in a play of fancy 

 quite inadmissible in the domain of natural science. 



It is admitted, however, that there is a correct idea at the base 

 of all this, for many species have not passed over into new forms, but 

 have simply died out because they were unable to adapt themselves 

 to changed conditions. This did not happen because they had 

 become incapable of variation, but because they could not produce 

 variations of sufficient magnitude, or variations of the kind required 

 to enable the species to remain an active competitor in the struggle 

 for existence. 



It obviously depends upon the coincidence of manifold circum- 

 stances, whether an adaptation can be successfully effected or not. 

 Above all, it must be able to keep pace with the changes in the 

 conditions of life, for if these advance at a more rapid rate the 

 organisms will succumb in the midst of the attempt at adaptation. 

 It is probably in this way that the striking disappearance of the 

 Trilobites is to be explained, as Neumayr has pointed out, for the 

 Nautilidae, a new group of enemies, multiplied so quickly at their 

 expense that they had not time to evolve any effective means of 

 protection. It cannot be maintained for a moment that every species 



