TRAXSMUTABLE. 121 



CHAPTER VII. 



I PASS on now to a further consideration of 

 Mr. Darwin's rule, as detailed in the first prop- 

 osition, and from which the necessary digression 

 about the apes has for a moment taken me. 

 Mr. 1). tells us this rule is of high generality, 

 but only applicable when the part, however un- 

 usually developed, is so in comparison with the 

 same part in allied species. Thus Mr. D. thinks 

 the bat's wing is a most u abnormal" structure 

 in the class mammalia, but his rule does not 

 apply to bats, because there is a whole group 

 of these animals with wings! Mr. D. gives one 

 example of the rule from the Cirripides, namely, 

 the opercular valves of the rock barnacles are in 

 every sense very important structures, and differ 

 little in different genera; but in Pyrgoma they 

 differ greatly, and the individuals of several of 

 the species differ more from each other in these 

 characters than species of distinct genera. 



Mr. Darwin says the rule applies strongly to 

 birds, but gives no examples. It fails however 

 in plants, because it is ''difficult to compare 

 their relative degrees of variability." 



Having given the above most meagre and un- 

 satisfactory reasons for believing in a law of the 

 greatest importance to his theory, Mr. Darwin 

 begins to generalize upon it, and turn it to ac- 



