362 THE BIOLOGY OF FLOWERING PLANTS 



important than scent, and they leave it a possibility that the 

 pigment of the flower is effective simply by reason of its 

 brightness, and not because of its colour as such. A great 

 deal of inconclusive work has been done on these points, e.g. 

 Plateau (1895, 6), Giltay (1904, 1906), Andreae (1903), Lovell 

 (1920). The questions, so far as the honey-bee is concerned, 

 have been settled recently by von Frisch (19 14). V. Hess 

 (191 3) had made an interesting comparison between the 

 colour sense of insects and other invertebrates and that of 

 colour-blind human beings. That very rare individual, a 

 totally colour-blind man, can distinguish no colours, though 

 he can distinguish between different light intensities. When 

 he views the solar spectrum he sees it shorter than we do — 

 the red end is merged in darkness ; he sees the brightest 

 light in the yellow-green region, while normal vision sees it 

 in the yellow. The relative intensities of different spectral 

 regions as seen by colour-blind and normal vision may be 

 measured and expressed as graphs, which differ in character- 

 istic ways. Von Hess found that this graph for bees and 

 other invertebrates, measured by making use of photo- 

 tropic response, corresponds with that of totally colour- 

 blind men. He drew the conclusion that these animals, 

 too, were totally colour-blind and could perceive only 

 differences in light intensity. If such were the case, the 

 flower would attract because it is bright and not because it 

 is, for example, blue. 



Von Frisch started out from the idea that this was not a 

 legitimate deduction from the experimental results, and 

 carried out experiments which seem to give conclusive 

 evidence in favour of a limited colour sense. If the bee 

 has no colour sense but distinguishes only intensities, then 

 it should be unable to distinguish between a colour and 

 that shade of grey which, to its senses, is of the same degree 

 of brightness. What that particular shade of grey in any 

 given case will be we cannot tell, but we can offer the insect 

 a series of greys, very finely graded, from which to choose. 

 Von Frisch used a series of 30 greys, in all shades from white 

 to black ; this grading was, as a matter of fact, finer than 



