86 THE DATA OF BIOLOGY. 



actions. The zoophyte rooted to a stone, and the acalephe 

 passively borne along in the current, need to undergo no 

 internal changes such as those by which the caterpillar meets 

 the varying effects of gravitation, wliile creeping over and 

 under the leaves. Again, the sea is liable to none 



of those extreme and rapid alterations of temperature which 

 the air suffers. Night and day produce no appreciable 

 modiiications in it ; and it is comparatively little affected by 

 the seasons. Thus its contained fauna show no marked cor- 

 respondences similar to those by which air-breathing crea- 

 tures counterbalance thermal changes. Further, in 

 respect to the supply of nutriment the conditions are more 

 simple. The lower tribes of animals inhabiting the water, 

 like the plants inhabiting the air, have their food brought to 

 them. The same current which brings oxygen to the oyster, 

 also brings it the microscopic organisms on which it 

 lives : the disintegrating matter and the matter to be inte- 

 grated, coexist under the simplest relation. It is otherwise 

 with land animals. The oxygen is everywhere ; but that 

 which is needed to neutralize its action is not everywhere : it 

 has to be sought ; and the conditions under which it is to be 

 obtained are more or less complex. So too with 

 that liquid by the agency of which the vital processes are 

 carried on. To marine creatures, water is ever present, and by 

 the lowest is passively absorbed ; but to most creatures living 

 on the earth and in the air, it is made available only through 

 those nervous changes constituting perception, and those 

 muscular ones by which drinking is effected. Simi- 

 larly, the contrast might be continued with respect to the 

 electric and hygrometric variations ; and the greater multi- 

 plicity of optical and acoustic phenomena with which ter- 

 restrial life is suri-ounded. And tracing upwards from the 

 amphibia the widening extent and complexity which the 

 environment, as practically considered, assumes — observing 

 further how increasing heterogeneity in the flora and fauna 

 of the globe, itself progressively complicates the environment 



