of certain littoral Mullusca. 387 



variation in the form, structure, or nature of tlie breathing organs them- 

 selves ; instead of looking for such difference elsewhere. And reasoning 

 analogically from the fact, that various branchiferous animals not only of 

 the MoUusca, but of much higher orders, such as Crustacea, various Fishes, 

 &c,, survive, as long as their branchicB only are kept moist, a total 

 deprivation of the fluid in which they habitually live immersed : while, 

 on the other hand, no well-established instances seemed recorded, of an 

 animal, with respiratory organs formed originally to derive oxygen im- 

 mediately from the atmospherick air, having the power of accommodat- 

 ing the same organs to the extraction of this vital principle from waterf : 

 it seemed more reasonable to consider, on the ground of my experi 

 ments, the above mentioned Mollusca to belong to the former of these 

 classes, than to regard them as indicating the existence of a new group 

 of animals possessed of the latter anomalous, and altogether unexampled, 

 characteristick. 



I am, however, perfectly aware of the danger in natural science of 

 carrying too far the argument from analogy, or of indulging too freely 

 in processes of generalization. I am sensible too of a deficiency in the 

 chain of facts ; and one that in most other countries I might have my- 

 self very easily supplied. I wish therefore at once to notice and account 

 for its omission. The point is thij ; my experiments prove indeed, that 



from the Preface. As to the observation on Hel. rtemoralis recorded in these 

 two last quotations, not to dwell upon the discrepancy or inaccuracy in the 

 accounts themselves of " plures dies" and " totCi restate," (for it can scarcely 

 be doubted that both statements refer to one and the same fact), no reliance can 

 be placed in a matter of such nicety on an observation which leaves it doubtful 

 whether an animal " in fundo rivi" or " in rivo," might not have frequent 

 opportunities of obtaining a supply of atmospherick air, though not actually 

 seen by Miiller himself in the act of doing so. 



* See inter alia, Mull. Verm. pp. 153, 160. 



•f The converse of this, it is well knov/n, is in some sense exemplified in the 

 Batrackia (amongst others) ; in the passage of the common Frog from the Tadpole 

 to the perfect state. But, in this case, it is not by the accommodation or modi- 

 fication of the old organ, but by the use of a distinct, coexistent, hitherto unem- 

 ployed one, that the animal at last breathes air, instead of water. 



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