THE "melanism" controversy. ■^00 



natural selection, or (8) tlie continuance of the conditions which 

 gave rise to them, may produce new species. (4) Isolation is 

 another secondary factor operating upon all variations. Mr. A. J. 

 Thomson defines the two principal schools of biological specu- 

 lation as follows : " A minority hold that the modification of 

 species takes place by cumulative growth, influenced by function 

 and environment, and pruned by natural selection; but the 

 majority hold that new species result from the action of natural 

 selection on numerous, spontaneous, and indefinite variations." 

 And he states as his opinion that " until we know much more 

 about the primary factors which directly cause variations, it will 

 not be possible to decide in regard to tlie precise scope of natural 

 selection, and the other secondary factors which foster and 

 accumulate, thin or prune ; vv'hich in short establish a new 

 organic equilibrium. The argument has been too much in 

 regard to possibilities, too little in regard to observed facts 

 of variation." If this be so in relation to zoology generally, it 

 seems more particularly to apply to those who have dealt with 

 the study of evolution among Lepidoptera, of course with the 

 exception of a few entomologists like Mr. Poulton and otliers, 

 who are most perseveringly experimenting as to the action 

 of immediate environment on certain species. I therefore 

 heartily endorse Mr. Sharp's strictures, taken in a general sense, 

 on the drift of the " melanic " discussion, and agree with him 

 that the best service entomologists can render to biological 

 science lies in the careful investigation, by exact experiment, 

 of the primary factors inducing variation. Fur as Weissmann 

 and others deny the transmissibiiity of changes produced by food, 

 temperature, &c., as well as those arising from functional causes, 

 use and disuse, &c., we should in the first place attack this 

 j)roblem, rather than take it for granted, as it seems to me 

 Mr. Tutt has done, and indeed most writers on the subject. For 

 if Weissmann's opinions are generally adopted, and they are 

 gaining much acceptance, theories based on the opposite belief 

 will be discredited. And workers in this field would do well not 

 to overlook the strongly-expressed opinion of Romanes, to the 

 effect that isolation is the universal condition to the establish- 

 ment of specific modification. Now, in the melanic or melano- 

 chroic variations said to be peculiar to smoke-discoloured 

 districts, where the factor of isolation does not seem to be 

 present, we apparently have to deal with a series of parallel 

 variations of various species which are said to be superseding the 

 type very rapidly. The question is whether this is effected by 

 natural selection, or, as many have argued, by the immediate 

 action of the environment. Before we can venture to form an 

 opinion in the absence of proof by experimental results, we 

 require reliable statistics as to the distribution of each of these 

 varieties, as to its spread locally in each district, and a list of the 



