352 CLASSIFICATION. [CHAP. XIV. 



piads having 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 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 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 ex- 

 ternal 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 shade and stripe of colour and even in the shape 

 of its wings, that Mr. Bates, with his eyes sharpened by collecting 

 during eleven years, was, though always on his guard, continually 

 deceived. When the mockers and the mocked are caught and 

 compared, they are found to be very different in essential structure, 

 and to belong not only to distinct genera, but often to distinct 

 families. Had this mimicry occurred in only one or two instances, 

 it might have been passed over as a strange coincidence. But, if 

 we proceed from a district where one Leptalis imitates an Ithomia, 

 another mocking and mocked species belonging to the same two 



