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May 7th 1851. 11

Against my theory, it may be urged & has often occurred to me, that there ought to be intermediate forms — but I forget not half-way between bird & Reptile, but some third form equally connected with both. — But there is another consideration, that from the manner in which we see groups in same order or class are connected, we shd expect not a form or intermediate in every character, but like in one part to the one side & in another organ to another.

And this at once give rise to extreme difficulty in knowing what would be intermediate. Look to the Proteo.— by Owen said by that greatest authority to be a true fish, but by many eminent zoologists to be a Reptile: this then

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cannot be far from an intermediate form.— If series be given to imagination, it will not be found very easy to make a form intermediate between any two classes— which wd be generally recognised from the as such, owing to the different importance attached by different naturalists to different parts.— Wd the ornithorhynchus have been kept as a mere division of the Marsupiata, if it had been covered with hairs feathers instead of hairs?

In fact mammalia, Fish, Reptiles are so far already connected, that, when we look at only

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the typical forms we expect that are intermediate form wd be something wonderfully different, which is erroneous—

Make classes of ornithorhynchus Labyrinthodon— Sauroid Fishes — Proteosaurus & perenne branchial Batrachian & & we shd have intermediate forms.— A great development as great as in Fish of these forms at the extreme of the classes wd almost break them down..— It wd be a marvelous chance if we were to hit upon the exact intermediate form & indeed they existed before Silurian system.—

To trust much to geological negative evidence is to trust to 2 or 3 volumes out of some score, devoted to history out of a large library destroyed by fire &c.—

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(I think the Classes of animals more specialised.— a Chonoid fish is eminently fish-like.— a brachynus crab [eminently] crab-like — a Balanus eminently cirripedial.— The record in Mammalia & Birds too imperfect.— The thecodont Reptiles not the most specialised — Labyrinthodon not the most Batrachian.—

Mollusca we do not go early enough

Insect far too imperfect.—