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318 THE WORLD OF LIFE 



on a green fruit which was repulsively bitter to myself. 

 Our nearest relatives among existing quadrupeds do not 

 therefore seem to have any need of a refined colour-sense. 

 Why then should it have been so highly developed in us ? 

 It was one of the fundamental maxims of Darwin that 

 natural selection could not produce absolute, but only 

 relative perfection ; and again, that no species could acquire 

 any faculty beyond its needs. 



The same arguments will apply even more strongly in 

 the case of insects. They appear to recognise the colours, 

 the forms, and the scents of flowers, but we can only 

 vaguely guess at the nature and quality of their actual 

 sensations. Their whole line of descent is so very far 

 removed from that of the birds that it is in the highest 

 degree improbable that there is any identity even in their 

 lower mental faculties with those of birds. For the colour- 

 sense is mental, not physical ; it depends partly on the 

 organ of vision, but more fundamentally on the nature 

 of the nervous tissues which transform the effects of light- 

 vibrations into the visual impressions which we recognise 

 as colour, and ultimately on some purely mental faculty. 

 But the colour-sense in insects may be quite other than 

 the bird's or than our own, and may in most cases be 

 combined with scent, and often with form to produce the 

 recognition of certain objects, which is all they require. 



Yet insects, birds, and the flowers and fruits which 

 attract them, all exhibit to our vision nearly the same 

 range of the colour-scheme, and a very similar intensity, 

 brilliancy, and purity of colour in particular cases ; which 

 is highly remarkable if their respective needs were the 

 only efficient causes in the production of these colours. 

 Looking first at flowers, how very common and conspicuous 

 are those of a yellow colour, yet far beyond the average 

 are the rich orange petals of the Escholtzia and the glisten- 

 ing splendour of some of our buttercups ; reds and purples 

 are innumerable, yet in the Lobelia fulgens and some other 

 flowers we reach an intensity of hue which seem to us 

 unsurpassably beautiful ; blues of the type of the cam- 

 panulas or the various blue liliaceae are all in their way 

 charming, but in the blue salvia (Salvia patens) the spring 





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