28 Colouration in Animals and Plants. 



direction has taken place. This is a most important conclusion, 

 though not very obvious at first sight. Still, when we bear in 

 mind the numbers of light waves of different lengths, and know 

 that if these are reflected irregularly, we get only mixed tints such 

 as indefinite browns ; we can at once see how, in the case of such 

 objects as tree trunks, and, still more, in inanimate things like rocks 

 and soils, these, so-to-say, undifferentiated hues are just what we 

 might expect to prevail, and that when definite colours are pro- 

 duced, it of necessity implies an effort of some sort. Now, if this 

 be true of such tints as red and blue, how much more must it be 

 the case with black and white, in which all the rays are absorbed 

 or all reflected? These imply an even stronger effort, and a priori 

 reasoning would suggest that where they occur, they have been 

 developed for important purposes by what may be termed a 

 supreme effort. Consequently, we find them far less common than 

 the others ; and it is a most singular fact that in mimetic insects, 

 these are the colours that are most frequently made use of. It 

 would almost seem as if a double struggle had gone on: first, the 

 efforts which resulted in the protective colouring of the mimicked 

 species, and then a more severe, because necessarily more rapid, 

 struggle on the part of the mimicker. 



Yet another point in this connection. If this idea be correct, 

 it follows that a uniformly coloured flower or animal must be of 

 extreme rarity, since it necessitates not merely the entire suppres- 

 sion of the tendency to emphasize important regions in colour, but 

 also the adjustment of all the varying parts of the organism to 

 one uniform molecular condition, which enables it to absorb all 

 but a certain closely related series of light waves no matter how 

 varied the functions of the parts. Now, such " self-coloured " 

 species, as florists would call them, are not only rare, but, 

 as all horticulturists know, are extremely difficult to produce. 

 When a pansy grower, for instance, sets to work to produce a self- 

 coloured flower — say a white pansy without a dark eye — his 

 difficulties seem insurmountable. And, in truth, this result has 

 never been quite obtained ; for he has to fight against every natural 

 tendency of the plant to mark out its corolla-tube in colour, and 

 when this is overcome, to still restrain it, so as to keep it within 

 those limits which alone allow it to reflect the proper waves of light. 



The production of black and white, then, being the acme of colour 

 production, we should expect to find these tints largely used for 



