intercross

There is no doubt that these three pheasants, namely, the common, the true ring-necked, and the Japan, intercross, and are becoming blended together in the woods of several parts of England.

From On the Origin of Species by Means of Natural Selection or the Preservation of Favoured Races in the Struggle for Life. (2nd edition) by Charles Darwin

If two distinct breeds were to be confined together in equal numbers, there is reason to suspect that they would to a certain extent prefer pairing with their own kind; but they would likewise intercross.

From The Variation of Animals and Plants Under Domestication, Volume II (of 2) by Charles Darwin

Mr. Walsh ranks the forms which it may be supposed would freely intercross, as varieties; and those which appear to have lost this power, as species.

From The Origin of Species by means of Natural Selection, 6th Edition by Charles Darwin

Phaseolus vulgaris is perfectly self-sterile; nevertheless, varieties growing in the same garden sometimes intercross largely.

From Effects of Cross and Self Fertilisation in the Vegetable Kingdom by Charles Darwin

He says, for example:— In the case of methodical selection, a breeder selects for some definite object, and if the individuals be allowed freely to intercross, his work will completely fail.

From Darwin, and After Darwin (Vol 3 of 3) Post-Darwinian Questions: Isolation and Physiological Selection by George John Romanes