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Hybridism

Take case of Pigeons paired for life —with all Equal fixed fertility — hardest case.—

Assume that country will support a given number.— But that with some sensible modification of structure & habit, a new species wd might be formed & be supported in additional numbers — As long as these new forms crossed

Each variety leading in this direction wd be preserved; but crossing would with old forms would constantly drag back to incipient form — now suppose a bird, varying slightly in desired direction was born, which when paired with its nearest ally, was fully fertile but was sterile with the old form; then its offspring which chanced to pair with its like would [illeg] be propagated, but those which propagated with the old would die out.— we shd have fewer

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of intermedial form, & more of the pure variety. Take case of a pigeon paired with similar var, producing offspring, fully fertile with own var. & not fully fertile (in either way) with distinct var. Would this new little family A have advantage in long run over other similar vars, which had no such tendency to sterility B? Many of As would die out from sterility would die out; a few would pair & leave pure new vars & these would strongly increase. Of B all the gradations would live, yet occasionally some purely matched vars would be produced. Occasionally perhaps all Bs would match with other vars & then new variety would be lost— so it would be with A!!! Perhaps matched pair if sterile would separate — My greatest difficulty with animals why mere sexual predilection does not by itself suffice; perhaps passion too strong.—

If no matching for life case wd be clear: prepotency clear with plants.—