A PRETTY LAND-SHELL. 179 



snails have two separate origins one large 

 group (including the garden-snail) being de- 

 rived from the common fresh-water mollusks, 

 while another much smaller group (including 

 the cycles toma) is derived from the opercu- 

 lated pond-snails. 



How is it, then, that naturalists had so 

 long overlooked this distinction ? Simply 

 because their artificial classification is based 

 entirely upon the nature of the breathing 

 apparatus. But, as Mr. Wallace has well 

 pointed out, obvious and important functional 

 differences are of far less value in tracing re- 

 lationship than insignificant and unimportant 

 structural details. Any water-snail may have 

 to take to a terrestrial life if the ponds in 

 which it lives are liable to dry up during 

 warm weather. Those individuals alone will 

 then survive which display a tendency to 

 oxygenise their blood by some rudimentary 

 form of lung. Hence the possession of lungs 

 is not the mark of a real genealogical class, 

 but a mere necessary result of a terrestrial 



N 2 



