THE ADAPTATION OF ORGANISMS 67 



and this is true in the case of the rabbits quoted above. 

 Numerous experiments have been carried on with refer- 

 ence to changes in the pressure of the environment, and 

 all have shown that pressure has a marked effect on the 

 organisms; for example, embryos have been known to 

 broaden out under artificial pressure, and it is an un- 

 doubted fact that water currents mould shells and corals, 

 and even water-leaves. 



Coming next to chemical influences, it is a well-known 

 fact that young animals of many kinds develop more 

 rapidly when well supplied with oxygen than they do 

 under normal circumstances. Further, it has been 

 proved experimentally that a deficiency of oxygen 

 tends to make life more sluggish, while an abundance 

 tends to increase the agility of the organism and the 

 rapidity of the life-processes. Nature herself proves 

 this daily by placing before our eyes the marvellous 

 activity of bird and insect, of air-inhabiting forms. The 

 presence or absence of water may cause considerable 

 modification. In plants excess of moisture is usually 

 accompanied by the absence of strengthening tissue, 

 while lack of water and consequently dryness of air 

 results in the formation of a thick external cuticle and 

 an abundance of skeletal tissue, while the entire absence 

 of water induces encystment in many of the lower 

 forms. An interesting example of the effect of water 

 or its absence is seen in the gorse. Young gorse seed- 

 lings have quite normal branches and typical leguminous 

 leaves. If these be cultivated in a moist atmosphere, 

 they develop into adults without any of the features 

 which we consider typical of the ordinary gorse plant. 



