Chap XTV.] ANALOGICAL RESEMBLANCES. 221 



with viscid discs, come under this same head of ana- 

 logical 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 in the growth or development of the parts, 

 and generally in their matured structure, can be 

 detected. The end gained is the same, but the means, 

 though appearing superficially to be the same, are 

 essentially different. The principle formerly alluded to 

 under the term of analogical variation has probably in 

 these cases 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 instance, 

 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 

 sinking the value of the groups in several classes (and 

 all our experience shows that their valuation is as yet 

 arbitrary), could easily extend the parallelism over a 

 wide range ; and thus the septenary, quinary, quater- 

 nary and ternary classifications have probably arisen. 



There is another and curious class of cases in whioh. 

 close external resemblance does not depend on adapta- 



