That experience is in
perfect accordance with this requirement is the more in favour of
Darwin, because the schoolmen far from being able to foresee or explain
such profound differences, must rather regard them as extremely
surprising. If, in the nearly allied families of the Ocypodidae and
Grapsoidae, the closest agreement prevails in all the essential
conditions of their structure; if the same plan of structure is
slavishly followed in everything else, in the organs of sense, in the
articulation of the limbs, in every trabecula and tuft of hairs in the
complicated framework of the stomach, and in all the arrangements
subserving aquatic respiration, even to the hairs of the flagella
employed in cleaning the branchiae,--why have we suddenly this
exception, this complete difference, in connection with aerial
respiration?
The schoolmen will scarcely have an answer for this question, except by
placing themselves on the theologico-teleological stand-point which has
justly fallen into disfavour amongst us, and from which the mode of
production of an arrangement is supposed to be explained, if its
"adaptation" to the animal can be demonstrated. From this point of view
we might certainly say that a widely gaping fissure which had nothing
prejudicial in it to Aratus Pisonii among the foliage of the mangrove
bushes, was not suitable to the Ocypoda living in sand; that in the
latter, in order to prevent the penetration of the sand, the orifice of
the branchial cavity must be placed at its lowest part, directed
downwards, and concealed between broad surfaces fringed with protective
brushes of hair.
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