When a mortgagee becomes entitled to possession as between himself and the mortgagor, he may maintain an action for possession against the mortgagor or the mortgagor's tenant under a lease made subsequently to the mortgage without the authority of the mortgagee or any other person not holding under a paramount title (a).

So long as the mortgagor or some one claiming under him is in actual possession, the mortgagee cannot maintain an action for trespass against a third party (b), but if the land is in a state of nature or has been left vacant by the mortgagor before a title of possession has been acquired by anyone, and the mortgagee has a right of entry, the constructive possession is in the mortgagee and he is entitled to bring an action for trespass (c).

After entry by a mortgagee his right of possession relates back to the time at which his legal right to enter accrued, so as to enable him to support an action against a wrong-doer for a trespass committed at a time antecedent to the entry (d).