GERMINAL SELECTION 135 



because of their femaleness, should vary in a different manner from 

 the males. 



But while I believe that sexual selection in particular has had 

 much to do with producing the colours of Lepidoptera, the basis of 

 all these colour variations must still be looked for in s'erminal 

 selection, and we shall see later on how it is possible to think of 

 the diversified and often relatively abrupt transformations of marking 

 as the resultant of the co-operation of climatic influences with o-erminal 

 selection. 



Of course there must also be unimportant changes in buttei*fly- 

 markings which depend solely on the internal play of forces in the 

 determinant system, and to this must be referred the markings of 

 many of the ' variable ' species whose variations are mere fluctuations 

 in the details of marking, which have therefore caused much trouble 

 to the systematists. Truly unimportant variations Avill rarely or 

 never combine into a ' constant ' form, and the fact that there 

 are species which are ' variable ' in such a high degree is enough 

 to make us refer their variations to their lack of importance, for 

 if they possessed any biological value the less valuable among them 

 would gradually be removed by selection. Perhaps the variable 

 species of certain moths like Arctia caja, and especially Arctia 

 2:)lantaginis, the little 'bear' of the Alps and Apennines, must 

 be reckoned among these. But from the fact that there are such 

 fluctuations in the markings of Lepidoptera, it seems to me that 

 we must conclude that species which show a high degree of con- 

 stancy in their markings have been influenced by selection, or 

 by climatic influences which turned the play of forces within the 

 determinant system in the same direction in all individuals. All 

 these considerations and conclusions are quite sound and serviceable 

 theoretically, but they are difficult to apply to individual cases, and 

 where this is attempted it must be with the greatest caution, and, 

 if possible, on a basis of investigations specially undertaken for the 

 purpose ; for how should we know whether a species which to-day 

 is highly variable may not a geological epoch later become a very 

 constant one ? We must in any case assume that marked fluctuations 

 of characters are associated with many transformations. 



