This is clearly shown by hybrids never having been raised between species ranked by systematists in distinct families; and on the other hand, by very closely allied species generally uniting with facility.

从未在不同科的系统学家分类的物种之间饲养杂交种清楚地表明了这一点；另一方面，亲缘关系非常密切的物种通常很容易结合在一起。