300 THE SIGNIFICANCE OF SEXUAL REPRODUCTION 



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 (Ortkoceras, 

 GomphoceraS) Goniatites, &c.) 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 themselves, 

 as a disproof of the sel f- variability of Nageli' s idioplasm ; for it must 

 be admitted that the facts are not sufficiently complete 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 J . 



In order to explain adaptation Nageli 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 perceptible amount, i. e. to a degree 

 which makes itself felt in visible external characters 2 . But such 

 changes alone could not be considered as adaptations, for the essen- 

 tial 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 are detrimental, the organism- attempts to defend itself 

 against the stimulus: a confluence of nutrient fluid takes place 

 towards the part upon which the stimulus has acted, and new tissues 



1 Appendix to pge 258. 2 1. c., p. 137. 



