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too, as we have seen in the case of plants, acceleiates the nutrition 

 and development of animal life. If the eggs of a frog be placed in 

 two glasses of water, one transparent and the other covered so as 

 to be impermeable to light, it -nill be found that in the first the 

 eggs wiU develop naturally, while those in the dark will develop 

 very slowly, and not advance fui-ther than rudimentary embryos. 

 Similar results will be obtained by placing the eggs of the common 

 house fly in glasses of different degrees of transparency or colour. 

 Placed simultaneously in various coloured glasses, insects of 

 different degi-ees of development will be produced. In an experi- 

 ment it was found that while all the eggs were hatched, the insects 

 in blue glasses were by far the most developed ; in gi-een they 

 were the smallest ; whilst in the red, yellow, and white were pro- 

 duced insects of ordinary size. Adult animals, too, are affected by 

 the colour of light. The effect of red light on the amount of 

 carbonic acid during respiration is well known. 



A word or two as to the effect of light in the formation of 

 colour in the skin. Formerly it was accepted without question that 

 all animal colouring was caused by the direct influence of light upon 

 the skin. Xow it has been clearly established by experiment that in 

 the tadpoles of common toads and frogs, the pigment is equally well 

 developed in total darkness, as in yellow, blue, or red light ; and the 

 development of pigment cells would seem to depend on influences 

 other than those arising from mere luminous rays. Such develop- 

 ments may be more closely connected with the heat producing, or 

 the chemical, not to add the electrical, influences associated with 

 sun-Hght. The theory of natural or sexual selection which is so 

 often used to account for differences in colour, would at fu-st sight 

 appear to be in antagonism to this suggestion ; but a careful con- 

 sideration will, I think, show that so far from this being the case, 

 the suggestion is supplementary only to the Darwinian principle. 



Man himself is conscious of the physical effect of bright sun- 

 shine; in addition to the beauty with which the sun in its glory 

 gilds aU nature, and paints the flowers, it wakens songs of joy in 

 the birds, and causes our own blood to flow with greater activity, 

 stimulating us to exertion and augmenting our enjoyment of life 

 itself. It is true all animals are not affected in the same way by 

 alternations of light and darkness. Some go to rest as night 

 approaches, others rouse themselves and go forth to seek their 



