of certain littoral Mollusca. 387 
variation in the form, structure, or nature of the 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 Mollusca, but of much higher orders, such as Crustacea, various Fishes, 
&e., survive, as long as their branchie 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 watert : 
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 Jatter anomalous, and altogether unexampled, 
characteristick. 
Iam, 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 this; my experiments prove indeed, that 
from the Preface. As tothe observation on Hel. nemoralis recorded in these 
two last quotations, not to dwellupon the discrepancy or inaccuracy in the 
accounts themselves of “ plures dies” and “ totd zstate,” (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. 
+ The converse of this, it is well known, is in some sense exemplified in the 
Batrachia (amongst others); in the passage of the common Frog from the Tadpole 
to the perfect state. But, in this case, itis not by the accommodation or modi- 
fication of the old organ, but by the use ofa distinct, coexistent, hitherto unem- 
ployed one, that the animal at last breathes air, instead of water. 
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