3o8 The Study of Animal Life part iv 



energy which pass in and out, neither can we understand 

 its higher hfe apart from its surroundings. 



To attempt a natural history of isolated. animals, whether 

 alive or dead, is like trying to study man apart from society. 

 For it is only when we know animals as they live and move 

 that we discover how clever, beautiful, and human they 

 are. Thus Gilbert White's Selbome is a natural history. ; and 

 therefore we began our studies with the natural life of 

 animals — their competition and helpfulness, their adaptations 

 to diverse kinds of haunts, their shifts and tricks, their 

 industries and their loves. 



At present, however, we have to do with the relation 

 between external and internal changes. We must find out 

 what the environment of an organism is, and what power it 

 has. In a smithy we see a bar of hot iron being hammered 

 into useful form. Around a great anvil are four smiths 

 with their hammers. Each smites in his own fashion as 

 the bar passes under his grasp. The first hammer falls, 

 and while the bar is still quivering like a living thing it 

 receives another blow. This is repeated many times till the 

 thing of use is perfected. By force of smiting one becomes 

 a smith, and by dint of blows the bar of iron becomes 

 an anchor. So is it with the organism. In its youth 

 especially, it comes under the influence of nature's hammers ; 

 it may become fitter for life, or it may be battered out of 

 existence altogether. Let us try to analyse the various 

 environmental factors. 



{a) Pressures. — First we may consider those lateral and 

 vertical pressures due to air or water currents and to 

 the gentle but potent force of gravity. The shriek of the 

 wind as it prunes the trees, the swish of the water as it 

 moulds the sponges and water-leaves, illustrate the tunes of 

 those pressure-hammers. Under artificial pressure embryos 

 have been known to broaden ; even the division of the egg is 

 affected by gravity ; water currents mould shells and corals. 

 The influence of want of room must also be noticed, for by 

 artificial overcrowding naturalists have slowed the rate of 

 development and reared dwarf broods ; and the rate of 

 human mortality sometimes varies with the size of the 



