The general rule is that if several parcels belonging to the same person, which are mortgaged to secure a single debt, become severed in title, they must, as between their respective owners, bear the mortgage debt rateably in proportion to their respective values (z), unless by special agreement in the mortgage itself or by declaration on the part of the mortgagor one parcel has been made liable to bear the debt in exoneration of the others (a).
Where however several parcels are mortgaged to secure the same debt, and the mortgagor afterwards conveys or mortgages to different persons, giving to each a covenant against incumbrances, then as between the purchasers or mortgagees the parcels are liable for the mortgage debt in succession, beginning with the property last sold (b).
(y) It is an essential feature of a true contract of suretyship that there should be in existence or contemplation an original or principal liability to which the liability of the surety is collateral. Lakeman v. Mountstephen, 1874, L.R. 7 H.L. 17; Petrie v. Hunter, 1884, 10 O.A.R. 127; Simpson v. Dolan, 1908, 16 O.L.R. 459. It is, however, a perversion of language to say that the liability of a mortgagor to the mortgagee is collateral to the liability of the purchaser of the mortgaged property. The principal liability is that of the mortgagor and the liability of the purchaser, if any, to the mortgagee is collateral to and based upon the liability of the mortgagor. Cf. Trust & Loan Co. v. McKenzie, 1896, 23 O.A.R. 167, at p. 170.
(z) Ker v. Ker, 1869, 4 I.R. Eq. 15, 25; In re Darby's Estate, Rendall v. Darby,  2 Ch. 465; Flint v. Howard,  2 Ch. 54.
(a) Marquis of Bute v. Cunynghame, 1826, 2 Russ. 275, 299; Lecnino v. Leonino, 1879, 10 Ch. D. 460, 465; In re Dunlop, Dunlop v. Dunlop, 1882, 21 Ch. D. 583, 588. As to the general rule and the application of the doctrine of marshalling, see 21 Halsbury, Laws of England, pp. 303-307.
The mortgagee or purchaser of one of the two parcels may be entitled to have the securities marshalled in order that effect may be given to his rights (c).
The doctrine of marshalling has been stated thus:-A mortgagee or other creditor having two funds to which he may resort shall not disappoint another creditor who can resort only to one of those funds; in such a case the court will marshal the funds, without regard to the interests of the debtor, so as to satisfy the claim of the creditor having both funds out of that fund which will leave a fund for the other creditors (d).
In Webb v. Smith (e) Cotton, L.J. states the doctrine thus:-
"If A. has a charge upon Whiteacre and Blackacre, and if B. also has a charge upon Blackacre only, A, must take payment of his 'charge out of Whiteacre, and must leave Blackacre, so that B., the other creditor, may follow it and obtain payment of his debt out of it: in other words, if two estates, Whiteacre and Blackacre, are mortgaged to one person, and subsequently one of them, Blackacre, is mortgaged to another person, unless Blackacre is sufficient to pay both charges, the first mortgagee will be compelled to take satisfaction out of Whiteacre, in order to leave the second mortgagee Blackacre, upon which alone he can go. There was also another class of cases in which the doctrine was applied, namely, where under the old law the creditors by simple contract had no claim upon real assets, unless charged with or devised for the payment of debts, a court of equity would compel specialty creditors who might resort in the first instance to the personal estate, in priority of creditors by simple contract, and to the real assets, in exclusion of them, to recover satisfaction in the first place out of the real assets as far as they went"( f).
(b) Jones v. Beck, 1871, 18 Gr. 671; see also Renwick v. Berry-man, 1886, 3 M.R. 387; In re Jones, Farrington v. Forrester,  2 Ch. 461. See also Sec. 139, infra.
(c) See Sec. Sec. 138 and 139, infra.
(d) Aldrich v. Cooper, Durham v. Lankester, Durham v. Armstrong, 1802, 18 R.C. 198; S.C. 18 Ves. 382, 1 W. & T.L.C. Eq. 35.
(e) 1885, 30 Ch.D. 192, at p. 200.
The singly-secured creditor cannot interfere with the right of the doubly-secured creditor to resort to any or either of the properties upon which he has security, but if the doubly-secured creditor realizes his claim out of the property upon which alone the single-secured creditor has a charge and thus renders such property unavailable to the latter, the latter is entitled to be subrogated to the place of the former against the property upon which originally the latter had no charge (g).