In general the benefit of covenants which cannot be likened to grants, and the burden of which does not fall on land, is confined to the covenantee and those who sustain his persona, namely, his executor or heir. In certain cases, of which the original and type was the ancient warranty, and of which the modern covenants for title are present examples, the sphere of succession was enlarged by the mention of assigns, and assigns are still allowed to represent the original covenantee for the purposes of that contract. But it is only by way of succession that any other person than the party to the contract can sue upon it. Hence the plaintiff must always be privy in estate with the covenantee. [406] C. It is impossible, however, to tell by general reasoning what rights will be held in English law to belong to the former class, or where the line will be drawn between the two. The authorities must be consulted as an arbitrary fact. Although it might sometimes seem that the test of the first was whether the service was of a nature capable of grant, so that if it rested purely in covenant it would not follow the land, /1/ yet if this test were accepted, it has already been shown that, apart from tradition, some services which do follow the land could only be matter of covenant. The grant of light and air, a well- established easement, is called a covenant not to build on the servient land to the injury of the light, by Baron Parke. /2/ And although this might be doubted, /3/ it has been seen that at least one well-established easement, that of fencing, cannot be considered as a right granted out of the servient land with any more propriety than a hundred other services which would be only matter of contract if the law allowed them to be annexed to land in like manner. The duty to repair exists only by way of covenant, yet the reasoning of the leading cases is drawn from the law of easement. On the other hand, a covenant by a lessee to build a wall upon the leased premises was held, in Spencer's Case, not to bind assigns unless mentioned; /4/ but Lord Coke says that it would have bound them if it had purported to. The analogy of warranty makes its appearance, and throws a doubt on the fundamental principle of the case. We can only say that the application [407] of the law is limited by custom, and by the rule that new and unusual burdens cannot be imposed on land.