BIOLOGY IN ITS WIDER ASPFXTS 1257 



its environment, and these may affect the germ-cells and the 

 progeny. But there is thrust as well as parry. The organisms are not 

 only modified by the environment ; they often select their environ- 

 ment. (8) Finally there is the sifting action of the environment. The 

 environment has been all through the ages a sifting agent, and 

 there has been an evolution of sieves as well as an evolution of the 

 material sifted. Thus we see how impossible it is to take a facile 

 view of the relations of the environment and the organism ; we have 

 mentioned eight kinds of influence and have not come to an end. 



It would take a long time to discuss the human environment, but 

 let us think for a little about some of the factors. Thus as to air, 

 much importance attaches to the efforts that one needs to make in 

 the beating of the heart and in the breathing movements to cope 

 with the slight quantitative differences that occur in the composition 

 of the air. When the oxygen is slighth^ less than usual the work of 

 the heart and lungs is greatly increased, and it is this additional 

 work that tires us, both adult and child. 



Then there is the importance of movement in the air. The work 

 of Prof. Leonard Hill has shown clearly that it is the circulation of 

 the air that is so vital. Not only does it prevent accumulation of 

 microbes, but it promotes evaporation from the skin; and for some 

 reason not at present very clear, the actual movement of the air 

 helps the respiratory exchange of gases. Thus one remembers how 

 Prof. Hill shut up certain students in two rooms, where the only 

 difference between them was that in one there was an apparatus 

 to keep the air moving. Those in the room with moving air were 

 least tired. 



Many volumes have already been written on light : the light that 

 kills microbes; the ultra-violet rays which, though they unfortu- 

 nately do not come through ordinary glass, act as a stimulant to 

 growth and development; the light which has a subtle psycho- 

 logical influence. How far-reaching is the light! 



As to warmth, it is not the difference of temperature that is so 

 important, because life is only possible within very narrow limits; 

 what is important is that when we get over-heated, for instance, 

 there are secondary consequences, which affect us deeply. If there 

 is over-heating and the sweat glands are not operating any too well, 

 difficulties arise, for harder work is demanded from the heart, and 

 so forth. This happens not only when the sweat glands are not 

 operating well, but if the clothes worn are too thick and heavy, if 

 free evaporation from the skin is prevented, and if the heat in the 

 body is not in circulation so to speak. There are many very serious 

 difficulties involved in over-heating. One can hardly forget Prof. 

 Leonard Hill's graphic disrobing of a curate — though over- 

 clothing is not confined to the clergy. The first thing he took off 

 was a wool-lined motor-coat; then a cardigan jersey; then a wool- 



