176 THE INDUCTIONS OF BIOLOGY. 



composition, in specific gravity, in temperature, in self-mohil- 

 ity. It is true that this generalization does not hold with com- 

 plete regularity. Organisms which are in some respects the 

 most strongl}^ contrasted with the environing inorganic world, 

 are in other respects less contrasted than inferior organisms. 

 As a class, mammals are higher than birds; and yet they are 

 of lower temperature and have smaller powers of locomotion. 

 The stationary oyster is of higher organization than the free- 

 swimming medusa; and the cold-blooded and less hetero- 

 geneous fish is quicker in its movements than the warm- 

 blooded and more heterogeneous sloth. But the admission 

 that the several aspects under which this increasing contrast 

 shows itself, bear variable ratios to each other, does not con- 

 flict with the general truth that as we ascend in the hierarchy 

 of organisms, we meet with not only an increasing differen- 

 tiation of parts but also an increasing differentiation from 

 the surrounding medium in sundry other physical attributes. 

 It would seem that this trait has some necessary connexion 

 with superior vital manifestations. One of those lowly 

 gelatinous forms, so transparent and colourless as to be with 

 difficulty distinguished from the water it floats in, is not 

 more like its medium in chemical, mechanical, optical, 

 thermal, and other properties, than it is in the passivity with 

 which it submits to all the influences and actions brought to 

 bear upon it; while the mammal does not more widely differ 

 from inanimate things in these properties, than it does in the 

 activity with which it meets surrounding changes by com- 

 pensating changes in itself. And between these extremes, 

 these two kinds of contrast vary together. So that in pro- 

 portion as an organism is physically like its environment it 

 remains a passive partaker of the changes going on in its 

 environment; while in proportion as it is endowed with 

 powers of counteracting such changes, it exhibits greater un- 

 likeness to its environment.* 



* This paragraph originally formed part of a review-article on '* Trans- 

 cendental Physiology," published in 1857. 



