388 THE STUDY OF ANIMAL LIFE CHAP. 



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 it is 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 the shape of an embryo may be altered ; even 

 the division of the egg is affected by gravity ; water 

 currants affect the growth of corals. The influence of want 

 of room must also be noticed, for, as we have mentioned, 

 dwarf broods of the water-snail Limnsea may be produced 

 in vessels which do not allow them sufficient exercise 

 ground on the surface. In the cases of the freshwater 

 snails, the influence of restricted space was separated off 

 from associated abnormal conditions, such as the accumu- 

 lation of poisonous waste-product ; but this is not always 

 practicable. 



(b) Chemical Influences. Quieter, but more potent, arc 

 the chemical influences which damp or fan the fire of 

 life, which corrode the skin or drug the system, which 

 fatten or starve, depress or stimulate. Along with these 

 we must include that most important factor food. 



When a lighted piece of tinder is placed in a vessel 

 full of oxygen it burns more actively. Similarly, super- 

 abundance of oxygen makes insects jump, makes the 

 simplest animals more agile, and causes the " phosphores- 

 cent ' lights of luminous insects to glow more brightly ; 

 and young creatures usually develop more or less rapidly 

 according as the aeration is abundant or deficient. The 



