II. THE RANGE OE VALIDITY OE TIIF. DOC- 

 TRINE OE MUTATION. 



§ 6. THE SIGNIFICANCE OF THE AVAILABLE EVIDENCE. 



Unlike the prevailing form of the theory of selection, 

 the doctrine of mutation lays stress on sudden or dis- 

 continuous changes, and regards only these as active in 

 the formation of species. The Darwinian form of the 

 theory of selection regards both these and Huctuating 

 \'ariations as oi)erati\'e in the origin of new forms, whilst 

 Wallace favors the other extreme, according to which 

 all formation of species goes by a slow and gradual 

 process of change. 



The two schools of thought naturally adopt dilterent 

 attitudes tow^ards tlie doctrine of mutation. It is at (Mice 

 rejected by Wallace's adherents, whilst those who in- 

 cline to Darwin's own form of the theory are less un- 

 reservedly inimical ; many of them have even greeted it 

 with open arms. 



Inasmuch as these two lines of thouglit liaxe been 

 clearly manifested in the critiques which have been ])ub- 

 lished of the first volume of this work. I ])ropose t«t dis- 

 cuss them briefly here, in order to ])()int out the funda- 

 mental questions which are involved in this controxersy. 



The extreme opponents of my thcorx- maintain tliat 

 there are no mutations at all : Xafura uou facif siilfns. 

 thev sav. What I have described as discontinuous changes. 



