THE STRUGGLE FOR EXISTENCE. 79 



are useless structures, functionless vestigial remnants. 

 They are now known to be of the greatest importance, 

 altering the composition of the blood or secreting sub- 

 stances essential for the regulation of the processes of 

 metabolism. He would be a rash man indeed who 

 would now assert that any part of the human body is 

 useless. 



The same may be said of the coloration of organisms, 

 and the rash statements so frequently made as to the 

 uselessness of the differences in colour which so often 

 distinguish species from each other. Doubtless colour 

 variations are not adaptive, and the colour differences 

 between species are not necessarily adaptive either. 

 Yet many possible uses of coloration have been brought 

 to light by the labours of Bates, Fritz Miiller, Wallace, 

 Poulton, and others, showing that it may often be of the 

 greatest importance in the struggle for existence ; as in 

 the case of the protective resemblance to surroundings 

 enabling an organism to escape from its enemies or to 

 approach its prey unnoticed ; of warning colours exhibited 

 by animals well able to defend themselves with poisonous 

 weapons ; of mimicry where a species gains advantage by 

 acquiring a resemblance to some distasteful or dangerous 

 form ; of recognition marks or sexual ornaments which 

 serve to bring the sexes together. An interesting ex- 

 periment on the selection value of colour differences was 

 performed by Di Cesnola on the praying mantis (Mantis 

 religiosa). This insect occurs in Italy in two varieties, 

 a green and a brown, adapted for concealment on green 

 or brown surfaces ; and it was found that green speci- 

 mens placed in brown surroundings and brown specimens 

 placed in green were invariably eaten by their enemies, 

 while individuals on a background which they matched 

 frequently escaped destruction. 



There was a difficulty much felt by the early advocates 

 of natural selection which has been removed now that 

 the process of inheritance is better understood. It was 



