3C6 SIGNIFICANCE OF SEXUAL REPRODUCTION [V. 



APPENDICES. 



Appendix I. Further Considerations which oppose 



Nageli's explanation of Transformation as 



DUE TO Internal Causes^ 



When I describe Nageli's theory of transformation as due to 

 active causes lying within the organism, as a phyletic force of 

 transformation, I do not mean to imply that it is one of those 

 mysterious principles which, according to some writers, consti- 

 tute the unconscious cause which directs the transformation of 

 species. Nageli's idioplasm, which changes from within itself, 

 is conceived as a thoroughly scientific, mechanically operating 

 principle. This cause is undoubtedly capable of theoretical 

 conception : the only question is whether it has any real exist- 

 ence. According to Nageli, the growing organic substance, the 

 idioplasm, not only represents a perpctiiwn mobile rendered 

 possible as long as its substance continually receives from 

 without the matter and force which are necessary for conti- 

 nuous growth, but it also represents a perpcttiiun vnriabile due 

 to the action of internal causes^. But this is just the doubtful 

 point, viz., whether the structure of the idioplasm itself compels 

 it to change gradually during the course of its growth, or 

 whether it is not rather the external conditions which compel 

 the ever slightly varying idioplasm to change in a certain 

 direction by the summation of small differences. It has been 

 shown above that we do not gain anything by adopting Nageli's 

 theory, because the main problem which organic nature offers 

 for our solution, viz. adaptation, remains unsolved. Hence this 

 theory does not explain the phenomena of nature, and I believe 

 that there are also certain facts which are directly antagonistic 

 to it. 



If the idioplasm really possessed the power of spontaneous 

 variability ascribed to it by Nageli ; if, as a result of its own 

 growth, it were compelled to undergo gradual changes, and 

 thus to produce new species, we should expect that the duration 



* Appendix to page 264. ^ 1. c., p. 118. 



