DEGREE OF LIFE VARIES AS DEGREE OF CORKESrONDENCE. 105 



The sea-anemone fixed to a stone, and the acaleph5 

 borne along in tlie current, need to undergo no internal 

 changes such as those by which the caterpillar meets the 

 varying effects of gravitation, while 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 modifications 

 in it; and it is comparatively little affected by the seasons. 

 Thus its contained fauna show no marked correspondences 

 similar to those by which air-breathing creatures counter- 

 balance 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 disinte- 

 grating matter and the matter to be integrated, co-exist under 

 the simplest relation. It is otherwise with land animals. 

 The oxygen is everywhere, but the sustenance is not every- 

 where : 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 consti- 

 tuting perception, and those muscular ones by which drinking 

 is effected. Similarly, after tracing upwards from the 

 Amphibia the widening extent and complexity which the 

 environment, as practically considered, assumes — after ob- 

 serving further how increasing heterogeneity in the flora and 

 fauna of the globe, itself progressively complicates the 

 environment of each species of organism — it might finally 

 be shown that the same general truth is displayed in the 

 history of mankind, who, in the course of their progress, 

 have been adding to their physical environment a social en- 



