298 THE SIGNIFICANCE OF SEXUAL REPRODUCTION 



APPENDICES. 



APPENDIX I. FURTHER CONSIDERATIONS WHICH OPPOSE NAGELI'S 



EXPLANATION OF TRANSFORMATION AS DUE TO INTERNAL CAUSES l . 



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, constitute 

 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 existence. According to Nageli, the 

 growing organic substance, the idioplasm, not only represents a 

 perpetuum mobile rendered possible as long as its substance con- 

 tinually receives from without the matter and force which are 

 necessary for continuous growth, but it also represents a per- 

 petuum variabile due to the action of internal causes 2 . 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 varia- 

 bility 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 of species, genera, 



1 Appendix to page 257. 

 " I.e., p. 118. 



