84 



certain that the change has, or rather is, taking place by 

 very gradual steps, and there is no suggestion of a sudden 

 mutation. 



Some butterflies vary greatly in their appearance at 

 different times of the year, so that they are protected by 

 their similarity to their surroundings in both the wet and 

 dry seasons, when conditions vary enormously. The differ- 

 ence is so great in some cases, that the wet and dry season 

 phases have been classed as different species. " Naturalists 

 were fairly astounded when, in 1898, Mr. Guy K. Marshall 

 first bred the black and blue dry season Precis sesamus 

 from the black and red P. natalensis. The two butterflies 

 differ in size, form, pattern, colours, relation of upper to 

 under surface, and habits." 1 This is not by any means a 

 solitary case among butterflies ; there are large numbers of 

 similar ones already described, and doubtless many that have 

 so far escaped observation. This is particularly the case in 

 tropical countries, where the wet and dry seasons produce 

 such differences in the surroundings, that what would be a 

 protective coloration in one season would be the reverse in 

 the other. Now a change in coloration might well take 

 place as a mutation, and it might be a favourable one 

 towards concealment. It does not seem probable that 

 mutations would take place in size, form, pattern, relation of 

 upper to under surface, and habits in the same individual 

 simultaneously. Moreover, these changes take place with 

 the utmost regularity every year, and are always the same 

 changes, except for the constant fluctuating variations in 

 individuals. Sports or mutations are comparatively rare, 

 and vary from each other just as much as fluctuating 

 variations. According to the mutation theory, it is difficult 

 to conceive of this alteration of forms being produced. By 

 the accumulation of small variations it seems explicable 

 enough. 



Protective seasonal changes of colour also occur among 

 mammals and birds in countries where the snow lies for a 



1 Poulton, op. cit., p. 208. 



