164 VIGNETTES FROM NATURE. 



and functional resemblances are the worst 

 possible guide to relationship by descent. 

 Almost all plants which grow under water 

 have very finely divided leaves ; but they are 

 all aberrant members of the most diverse and 

 widely separated families. They have been 

 compelled to acquire long, thin, waving 

 leaflets because there was so little carbonic 

 acid in the water where they lived that they 

 could not extract carbon enough from it to 

 build up a full, large, round type of leaf; 

 exactly as all aquatic animals have much- 

 branched gills to catch the stray floating 

 particles of free oxygen dissolved in the 

 water, while all land-animals have big in- 

 ternal lungs, into which the abundant free 

 oxygen of the atmosphere pours copiously at 

 every inhalation. Put any two distinct 

 groups of plants or animals into exactly 

 similar circumstances, and the chances are 

 that they will adapt themselves to those 

 circumstances in exactly similar ways, thus 

 masking their original unlikeness. But if you 



