FUNCTION AND ENVIRONMENT 193 



our science, paralyzed its thinkers, some- 

 times even skeletonized or mummified them. 

 There is an endless diversity in environments, 

 and some of them are most extraordinary — 

 the iceberg, the hot spring, the mountain top, 

 the abysses of the ocean, the cave, the in- 

 terior of another creature — but for each 

 kind of organism there is an indispensable 

 minimum of supplies and influences, apart 

 from which it cannot develop, or grow, or 

 continue to live. This is the fundamental 

 relation of living things, that of constant 

 and normal environmental dependence. 



(2) But surroundings are changeful and 

 the living creature changes with them. In 

 many cases, where the external changes are 

 regularly recurrent, like seasons and tides, 

 the organism falls into step with them; so 

 that there are internal rhythms, punctuated 

 by external periodicities. The latter may 

 come to be needed only as the liberating 

 stimuli, or trigger-pullers, of the former. 

 Experiments show that some young tropical 

 acacias are hereditarily wound up, as it 

 were, to a twelve hours' day and night — - 

 times of leaf-expansion and leaf-closure. 

 The cold of winter is probably the stimulus 

 rather than the efficient cause of the brown 

 stoat becoming the white ermine. 



(3) To some of the irregular changes in its 



