The land itself vomiteth out her inhabitants - This is a very nervous prosopopoeia or personification; a figure by which any part of inanimate nature may be represented as possessing the passions and reason of man. Here the land is represented as an intelligent being, with a deep and refined sense of moral good and evil: information concerning the abominations of the people is brought to this personified land, with which it is so deeply affected that a nausea is produced, and it vomits out its abominable and accursed inhabitants. It was natural for the inspired penman to make use of such a figure, as the description he was obliged to give of so many and enormous abominations must have affected him nearly in the same way in which he represents the land to be affected.

And the land is defiled: therefore I do (m) visit the iniquity thereof upon it, and the land itself (n) vomiteth out her inhabitants.
(m) I will punish the land where such incestuous marriages and pollutions are tolerated.
(n) He compares the wicked to evil humours and overeating, which corrupt the stomach, and oppress nature, and therefore must be cast out by vomit.

And the land is defiled,.... The inhabitants of it, with the immoralities and idolatries before mentioned:
therefore I do visit the iniquity thereof upon it; or punish the inhabitants that are on it for their sins:
and the land itself vomiteth out her inhabitants; as a stomach loaded with corrupt and bad food it has taken in, nauseates it, and cannot bear and retain it, but casts it up, and never receives it again; so the land of Canaan is represented as loathing its inhabitants, and as having an aversion to them, and indignation against them, and as not being able to bear them, but entirely willing to be rid of them and throw them out of their places in it, never to be admitted more, being as nauseous and as useless as the cast of a man's stomach; see Revelation 3:16.

therefore I do visit the iniquity thereof upon it; and the land itself vomiteth out her inhabitants--The Canaanites, as enormous and incorrigible sinners, were to be exterminated; and this extermination was manifestly a judicial punishment inflicted by a ruler whose laws had been grossly and perseveringly outraged. But before a law can be disobeyed, it must have been previously in existence; and hence a law, prohibiting all the horrid crimes enumerated above--a law obligatory upon the Canaanites as well as other nations--was already known and in force before the Levitical law of incest was promulgated. Some general Iaw, then, prohibiting these crimes must have been published to mankind at a very early period of the world's history; and that law must either have been the moral law, originally written on the human heart, or a law on the institution of marriage revealed to Adam and known to the Canaanites and others by tradition or otherwise.

I visit - I am about to visit, that is, to punish.