XVII. j THE PROBLEM OF THE ORIGIN OF SPECIES. 215 



the varieties of those species, is, I think, only a particular Extension 

 case of a much wider law, which may be thus stated : — above^- 

 The wider a group to ivhich any cliaracter helongs, the less is stated law 

 that character liable to excc])tions. Tlitis, for instance, jointed tion. 

 legs are a character of all the higher Articulata, and they 

 are found in every species of all their classes, without a 

 single exception.^ Wings, on the contrary, are found only in Wings of 

 the class of true or hexapod insects ; and, though general, ^^^^^ ''' 

 they are by no means constant in their class. Another 

 instance of this kind is that of the respiratory organs in the Branohiai 

 three allied molluscan classes of Gasteropoda, Heteropoda, "^ "^°1" 

 and Pteropoda. Most members of these classes have dis- 

 tinct respiratory organs ; but their occurrence is not uni- 

 versal, as some of them respire through the general surface 

 of the body. When they do occur, their position is very 

 variable as between the various families. In Firoloidcs 

 Des^narcstii, a heteropod, there are none, though they occur 

 in some nearly allied species ; and in one of these, by 

 name Atlanta Lesuerii, some individuals have them, and 

 others not.^ Another instance of the same kind is that of Develop- 

 the extraordinary larval forms presented by some of the ^"Egiiiuo. 

 Echinodermata in the course of their development. This dermata. 

 peculiar mode of development, of which I shall have to 

 speak in a future chapter, " in each order appears to be 

 exceptional ; and in certain cases it is known to be carried 

 to its most abnormal degree in one species, while in a 

 closely allied species of the same genus the mode of repro- 

 duction differs but slightly from the ordinary invertebrate 

 type. It seems highly probable that even in the same 

 species the development and independence of the first 

 zooid may be carried to a greater or less degree according 

 to circumstances."^ 



The other law which we have seen to be established, ^ ^ . 



JLxtension 



that varieties tend to revert to the characters of the of the 



1 Except, perhaps, among the lowest of the acaii, or mites. 



2 Huxley on the Morphology of the Cephalous Mollusca (Philosophical 

 Transactions, IS.IS). 



3 Dr. "Wyvillc Thomson on the Etymology of the Echinodermata 

 (Natural History Review, July 1863). 



