The Senses of Animals. 285 



centre green is the predominant colour of the cones, while 

 among the green cones red and orange ones are somewhat 

 sparingly interspersed, and are nearly always arranged 

 alternately — a red cone between two orange ones, and vice 

 versa. In a surrounding portion, called by Dr. Waelchli 

 the red zone, the red and orange cones are arranged in 

 chains, and are larger and more numerous than near the 

 yellow spot ; the green ones are of smaller size, and fill up 

 the interspaces. Near the periphery the cones are scattered, 

 the three colours about equally numerous and of equal 

 size, while a few colourless cones are also seen. Dr. 

 Waelchli examined the optical properties of the coloured 

 cones by means of the micro-spectroscope, and found, as 

 the colours would lead us to suppose, that they transmitted 

 only the corresponding portions of the spectrum ; and it 

 would almost seem, excepting for the few colourless cones 

 at the peripheral part of the retina, that the birds examined 

 must have been unable to see blue, the whole of which 

 would be absorbed by their colour-globules." * 



These facts are of exceeding interest. They seem to 

 show that for these birds the retinal explosives are not the 

 same as for us. They are B„, 0., and Gr. Moreover, the 

 colour-globules will have the effect of excluding the pheno- 

 mena of overlapping. For each kind of cone the spectrum 

 must be limited to the narrow spectral band transmissible 

 through the associated colour-globule. If these facts be 

 so, it is not too much to say that the colour-vision of birds 

 must be so utterly different from that of human beings, 

 that, being human beings, we are and must remain 

 unable to conceive its nature. The factors being different, 

 and the blending of the factors by overlap being, by 

 specially developed structures, lessened or excluded, the 

 whole set of resulting phenomena must be different from 

 ours. And this is a fact of the utmost importance when 

 we consider the phenomena of sexual selection among 

 birds, and those theories of coloration in insects which 

 involve a colour-sense in birds. 



* " Col our- Vision and Colour-Blindness," E. Brudenell Carter (Nature, 

 vol. slii. p. 56). 



