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12

Embryology

If 1/100 part of each variations affect very young embryo, then the greater the amount of variation which one of 2 descendants from one organism undergoes, so much the more will the embryo be earlier affected. Hence the embryo of mammal will sooner be differentiated, from the common type or parent, than will a fish.

[in margin:] Better omit this too difficult.

But I do not suppose things go on so simply. It may be (mere hypothesis) that each change of adult affected in some degree young, & that each change was superimposed on to last & did not necessarily obliterate it, if a beak was first rendered very long it wd slightly effect embryo & then made short again that the change would not leave the embryo as it was originally, but wd first give it a longer & then a shorter beak.—

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The foregoing wd agree with Brulle's law & Barneouds.

From the foregoing the parts which are little modified in embryo, are so from not having suffered so much modification in the mature, hence, these are the most constant characters & ∴ of most value to classification.

No this is not same as Milne Edwards, he asserts that the parts which appear first, whether or not modified, are the best for classification, & are ∴ most constant in their existence. On the superimposed doctrine, a heart which appears late in embryo, is a part which has been formed by modification of some other part late in the

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(2

Branchiæ I do not doubt first created in Vertebrate types & then swim-bladder but when was that created? metamorphosed into lungs, & it seems (Annl. des Sc. Nat 3d series Zoology p. 22 of my Abstract) the branchiæ in embryo appear before swim-bladder or lungs in Batrachians.

 

genesis of the form & therefore would not be so valuable as a character which was inherited from the earliest dawn of group. The superposition doctrine wd agree with lungs & branchiæ of Batrachian, but it wd flatly contradict Brulle & Barneouds law, in which greatly developed parts appear earlier than parts less modified.

N.B. The appearance of a part, the heart of a Vertebrate or of Mollusca, in many the medication of some other part.