30 THE DATA OP BIOLOGY. 



liquids which the tissues already contain. Hence, to the 

 extent that such escape is facilitated by evaporation, and this 

 evaporation facilitated by heat, heat becomes an agent of re- 

 distribution in the animal organism.* 



13. Light, which is now known to modify many inor- 

 ganic compounds light, which works those chemical changes 

 utilized in photography, causes the combinations of certain 

 gases, alters the molecular arrangements of many crystals, 

 and leaves traces of its action even on substances that are 

 extremely stable, may be expected to produce marked effects 

 on substances so complex and unstable as those which make 

 up organic bodies. It does produce such effects; and some 

 of them are among the most important that organic matter 

 undergoes. 



The molecular changes wrought by light in animals are 

 of but secondary moment. There is the darkening of the 

 skin that follows exposure to the Sun's rays. There are 

 those alterations in the retina which cause in us sensations 

 of colours. And on certain eyeless creatures that are semi- 

 transparent, the light permeating their substance works some 

 effects evinced by movements. But speaking generally, 

 the opacity of animals limits the action of light to their 

 surfaces; and so renders its direct physiological influence 



* The remark made by a critic to the effect ihat in a mammal higher tem- 

 perature diminishes the rate of molecular change in the tissues, leads me to 

 add that the exhalation I have alleged is prevented if the heat rises above the 

 range of variation normal to the organism ; since, then, unusually rapid pulsa- 

 tions with consequent inefficient propulsion of the blood, cause a diminished 

 rate of circulation. To produce the effect referred to in the text, heat must 

 be associated with dryness ; for otherwise evaporation is not aided. General 

 evidence supporting the statement I have made is furnished by the fact that 

 the hot and dry air of the eastern deserts is extremely invigorating; by the 

 fact that all the energetic and conquering races of men have come from the 

 hot and dry regions marked on the maps as rainless ; and by the fact that 

 travellers in Africa comment on the contrast between the inhabitants of the 

 hot and dry regions (relatively elevated) and those of the hot and moist 

 regions : active and inert respectively. 



