.Chap. XIV. A nalog ical Resent blances. 375 



in either case have been adapted for tearing flesh, through the 

 natural selection of successive variations ; but if this be admitted 

 in the one case, it is unintelligible to me that it should be denied 

 in the other. I am glad to find that so high an authority as 

 Professor Flower has come to this same conclusion. 



The extraordinary cases given in a former chapter, of widely 

 different fishes possessing electric organs, — of widely different 

 insects possessing luminous organs, — and of orchids and asclepiads 

 havin * pollen-masses with 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 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 dif- 

 ferent. The principle formerly alluded to under the term of ana- 

 logical 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, inde- 

 pendently 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 parallelism has sometimes been observed between the 

 sub-groups of distinct classes. A naturalist, struck with a paral- 

 lelism 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 paral- 

 lelism 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 which close external 

 resemblance does not depend on adaptation to similar habits of life, 

 but has been gained for the sake of protection. I allude to the 

 wonderful manner in which certain butterflies imitate, as first 

 described by Mr. Bates, other and quite distinct species. This 

 excellent observer has shown that in some districts of S. America, 

 where, for instance, an Ithomia abounds in gaudy swarms, another 

 butterfly, namely, a Leptalis, is often found mingled in the same 

 flock; and the latter so closely resembles the Ithomia in every 



