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Jan 4 1863 An animal, say, becoming adapted for aquatic life & surrounded by so many of its parent terrestrial forms, that in course of few generations all such vars, chanced to get crossed by them; would be dragged back from its favourable variation.

But if a variation arose, which was sterile with ordinary form, then although ever so large a proportion was united with ordinary form & was ultimately lost by sterility; yet if ever so few united with similar form, these would be kept pure; but then chances are supposed that they would unite with ordinary form & so would be lost by sterility. It will not do. Then sterility must supervene for being kept close to uniform conditions of life for ages.

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If sexual disinclination supervened all would go well; but then why does sterility supervene, & why in Hybrid & not always in first cross — — will not do. —

N. B Sexual disinclination must be an acquired instinct: for how else could it arise?