THE ORIGIN AND THE EXTINCTION OF SPECIES 355 



of species to vary persistently onwards, and to ' become furtlier and 

 further removed from the primitive type,' as has been maintained, 

 then such ancient and primitive Cephalopod forms like the Naiitilus- 

 species could not have persisted until now, but must long ago have 

 been transmuted into higher forms. The converse, however, is con- 

 ceivable enough, namely, that the great mass of the species of a group 

 such as the Xautilida? were crowded out by superior rivals in the 

 struggle for existence, but that certain species were able to survive 

 cm specially protected or otherwise favoured areas. We have a fine 

 example of this in the few still living species of the otherwise 

 extinct class of Ganoid hshes. During the Primary and Secondary 

 epochs these Ganoids peopled all the seas, but at the boundar}^ 

 between the Cretaceous and the Tertiary period they retrograded 

 considerably, simultaneously with the great development of bon}- 

 fishes or Teleosteans, and now they are only represented by a dozen 

 species distributed over the earth, and most of these are purely river 

 forms, while the others at least ascend the rivers during the spawning 

 season to secure the safety of their progeny. For the rivers are 

 sheltered areas as compared with the seas, and large fishes like the 

 Ganoids will be able there to hold their own in the struggle better 

 than they could in the incomparably more abundantly peopled sea. 



■ Thus I can only regard it as playing with ideas to sj)eak of birth, 

 blossoming, standstill, decay, and death of species in any other than 

 a figurative sense. Undoubtedly the life of the species may be 

 compared with that of the individual, and if the comparison be used 

 only to make clear the difference between the causes of the tAvo kinds 

 of phenomena, there can be no objection to it, only we must beware 

 of thinking we have explained anything we do not know by comparing 

 it with something else that is also unknown. 



We have already shown that the natural death of multicellular 

 organisms is a jDhenomenon which first made its appearance Avith the 

 separation of the organism into somatic or body cells and reproductive 

 or germ cells, and that death is not an inevitable Xemesis of every life, 

 for unicellular organisms do not necessarily die, though they may 

 be killed by violence. These unicellular organisms have thus no 

 natural death, and we have to explain its occurrence among multi- 

 cellular organisms as an adaptation to the cellular differentiation, 

 which makes the unlimited continuance of the life of the whole 

 organism unnecessary and purposeless, and even prejudicial to the 

 continuance of the species. For the species it is enough if the germ- 

 cells alone retain the potential immortality of the unicellulars, while, 

 on the other hand, the high differentiation of the somatic cells neces- 



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