ECOLOGICAL 



49 



there is an indispensable minimum of supplies and influences (such 

 as oxygen, water, food, warmth), apart from which it cannot 

 develop, or grow, or continue to live. 



(2) But environments are changeful and organisms must change 

 with them; and in many cases there has come to be an established 

 attunement between the living creature and the outside changes, 

 such as the ebb and flow of the tide, the alternation of day and 

 night, or the march of the seasons. There are ingrained or hereditary 

 rhythms, which have become synchronous with external periodici- 

 ties. The outside events serve rather as liberating stimuli than as 

 direct causes. 



(3) To particular changes in the environment the individual 

 living creature often answers back — even a tree to a passing cloud, 

 and an earthworm to the vibrations in the soil caused by the tread 



AECHANICAL 

 CHEMICAL 

 PHYSICAL 

 ANirViTE 



Fig. 



15. 



The Organism, with its three germinal layers (ectoderm, mesoderm, and 

 endoderm) (i, 2, 3), is represented as surrounded by environmental in- 

 fluences — mechanical, chemical, physical, and animate — some of which 

 penetrate further than others, as the various lettered arrows suggest. 



of many feet. These are immediate responses or reactions, and 

 sometimes they are worthy of being called adjustments, as when the 

 warm-blooded bird or mammal on a very cold day automatically 

 regulates its production of heat to compensate for its unusual loss. 

 Even subtler is the way in which animals with a power of rapid 

 colour-change, e.g. fiat-fishes, adjust their coloration to suit the 

 gravel or sand on which they are resting, thus sometimes securing 

 protection by unconsciously making themselves invisible. 



(4) When some peculiarity in the environment impresses a lasting 

 change on the organism, that is called a modification. The indent 01 

 imprint transcends the limits of organic elasticity and persists after 

 the inducing causes have ceased to operate. As examples may be 

 mentioned : Sun-burning in the course of a long holiday, or thicken- 



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