Chap. XIV.] ANALOGICAL RESEMBLANCES. 221 



viscid discs, come under this same head of analogical 

 resemblances. But these cases are so wonderful that they 

 were introduced as difficulties or objections to our 

 theory. In all such cases some fundamental difference 

 m the growth or development of the parts, and gen- 

 erally in their matured structure, can be detected. The 

 end gained is the same, but the means, though appear- 

 ing superficially to be the same, are essentially different. 

 The principle formerly alluded to under the term of 

 analogical variation has probably in these eases often 

 come into play; that is, the members of the same class, 

 although only distantly allied, have inherited so much 

 in common in their constitution, that they are apt to 

 vary under similar exciting causes in a similar manner; 

 and this would obviously aid in the acquirement through 

 natural selection of parts or organs, strikingly like each 

 other, independently of their direct inheritance from a 

 common progenitor. 



As species belonging to distinct classes have often 

 been adapted by successive slight modifications to live 

 under nearly similar circumstances, — ^to inhabit, for in- 

 stance, the three elements of land, air, and water,- — we can 

 perhaps understand how it is that a numerical paral- 

 lelism has sometimes been observed between the sub- 

 groups of distinct classes. A naturalist, struck with a 

 parallelism of this nature, by arbitrarily raising or sink- 

 ing the value of the groups in several classes (and all 

 our experience shows that their valuation is as yet arbi- 

 trary), could easily extend the parallelism over a wide 

 range; and thus the septenary, quinary, quarternary and 

 ternary classifications have probably arisen. 



There is another and curious class of cases in which 

 close external resemblance does not depend on adapta- 



