202 THE BIOLOGY OF THE SEASONS 



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" (The Study of Animal Life, p. 308). 



It seems convenient to begin with a set of influences 

 which may be associated together under the title molar or 

 mechanical. We refer to pressures lateral and vertical, 

 to currents, to the action of gravity, to the amount of 

 available space. 



Many years ago, Professor Semper took a number of 

 young freshwater snails (Limncea stagnalis), apparently 

 identical, and put them in different glass vessels, with the 

 same kind of water, the same kind of food, and so on, the 

 only known difference being that the volume varied. After 

 two months it became plain that they were not getting on 

 equally well ; those in the vessels of large volume were 

 growing apace, those in the small vessels were beginning to 

 lag behind. As others have since done, Semper reared a 

 series of dwarfs. Yung has also shown that the rate of 

 growth of tadpoles bears some relation to the ratio of the 

 amount of space to the number occupying it. Probably 

 there was some other factor besides the amount of room, but 

 there was no lack of food, air, or cleansing. In De Varigny's 

 repetition of Semper's experiments the conclusion arrived at 

 was, that the amount of exercise-ground at the top of the 

 water was a very important factor. 



The trend of the branches on a tree often shows the 

 direction of the prevailing wind, and on some exposed parts 

 of our coast the surface of the tree-tops in the wood slopes 



