Characters, Hereditary and Acquired. 55 



stance ; but as soon as we begin to cast about for 

 cases which will satisfy the Neo-Darwinians, we find 

 that the structure of their theory is such as to pre- 

 clude, in almost every conceivable instance, the possi- 

 bility of meeting their demand. For their theory begins 

 by assuming that natural selection is the one and only 

 cause of organic evolution. Consequently, what their 

 demand amounts to is throwing upon the other side 

 the burden of disproving this assumption — or, in other 

 words, ofproving the negative that in any given case of 

 transmitted adaptation natural selection has not been 

 the sole agent at work. Now, it must obviously be 

 in almost all cases impossible to prove this negative 

 among species in a state of nature. For, even sup- 

 posing that among such species Lamarckian prin- 

 ciples have had a large share in the formation of 

 hereditary and adaptive characters, how would Weis- 

 mann himself propose that we should set about the 

 proof of such a fact, where the proof demanded by his 

 assumption is, that the abstract possibility of natural 

 selection having had anything to do with the matter 

 must be excluded.? Obviously this is impossible in 

 the case of inherited characters which are also 

 adaptive characters. How then does it fare with the 

 case of inherited characters which are not also 

 adaptive ? Merely that this case is met by another 

 and sequent assumption, which constitutes an integral 

 part of the Neo-Darwinian creed — namely, that in 

 nature there can be no snch characters. Seeing that 

 natural selection is taken to be the only possible 

 cause of change in species, it follows that all changes 

 occurring in species must necessarily be adaptive, 

 whether or not we are able to perceive the adaptations. 



