308 SIGNIFICANCE OF SEXUAL REPRODUCTION [V. 



rest in the process of change which the idioplasm must un- 

 dergo ; and this is as true of each single species as it is of 

 the organic world taken as a whole. We could, perhaps, find 

 shelter in the insufficiency of our geological knowledge, but the 

 number of ascertained facts is too great for this to be possible. 

 Thus it is well known that the genus Nautilus has lasted from 

 Silurian times, through all the three geological periods, up to 

 the present day; while all its Silurian allies {Orthoceras, Gom- 

 phoceras, Gouiatitcs, etc.) became extinct at a comparatively 

 early period. 



A keen and clever controversialist might still bring forward 

 many objections against such an argument. I do not therefore 

 place too much dependence upon the geological facts by them- 

 selves, as a disproof of the self- variability of Nageli's idioplasm ; 

 for it must be admitted that the facts are not sufficiently com- 

 plete for this purpose. For instance, in the case of Nautilus 

 it might be argued that we do not know anything about the 

 fossil Cephalopods of pre-Silurian times, and that it is therefore 

 possible that the above-mentioned allies of Nautilus may have 

 existed previously for as long a period as that through which 

 Nautilus has lived in post-Silurian time. However this may 

 be, it will be at least conceded that the geological facts do not 

 lend any support to Nageli's theory, for we can see no trace 

 of even an approximately regular succession of forms. 



Appendix II. Nageli's explanation of Adaptation\ 



In order to explain adaptation Nagcli assumes that, under 

 certain circumstances, external influences may cause slight 

 permanent changes in the idioplasm. If then such influences 

 act continually in the same direction during long periods of 

 time, the changes in the idioplasm may increase to a percep- 

 tible amount, i.e. to a degree which makes itself felt in visible 

 external characters-. But such changes alone could not be 

 considered as adaptations, for the essential character of an 

 adaptation is that it must be a purposeful change. Nageli, 

 however, brings forward the fact that external stimuli often 

 produce their chief effects at that very part of the organism 

 to which the stimuli themselves were applied. ' If the results 



' Appendix to page 264. - 1. c, p. 137. 



