ANALOGICAL RESEMBLANCES. 415 



rariations ; 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 having pollen-masses with viscid disks, come 

 under this same head of analogical resemblances. But these 

 cases are so wonderful that they were introduced as difficul- 

 ties 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 

 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 obvi- 

 ously 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 parallelism has some- 

 times 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 sev- 

 eral 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, quaternary, 

 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 simi- 

 lar habits of life, but has been gained for the sake of pro- 

 tection. 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 South America, where, for instance, 

 an Ithomia abounds in gaudy swarms, another butterfly, 



