Besides writers referred to above "some foreign writers, in particular Scilla and Rouelle, appear to have made very just comparisons of the natural associations of fossil shells, corals, &c. in the earth, with the groups of similar objects as they are found in the sea, and thus to have produced new proofs of the organic origin of these fossil bodies; but they give no sign of any knowledge of the _limitation of particular tribes of organic remains to particular strata_, of the _successive existence of different groups of organization_, on _successive beds of the antient sea_. Mr Smith's claim to this happy and fertile induction is clear and unquestionable[3]." We get a clue to the manner in which he arrived at his view in the following passage[4]:--"Accustomed to view the surfaces of the several strata which are met with near Bath uncovered in large breadths at once, Mr Smith saw with the distinctness of certainty, that 'each stratum had been in succession the bed of the sea'; finding in several of these strata abundance of the exuviae of marine animals, he concluded that these animals had lived and died during the period of time which elapsed between the formation of the stratum below and the stratum above, at or near the places where now they are imbedded; and observing that in the successively-deposited strata the organic remains were of different forms and structures--Gryphites in the lias, Trigoniæ in the inferior oolite, hooked oysters in the fuller's earth,--and finding these facts repeated in other districts, he inferred that each of the separate periods occupied in the formation of the strata was accompanied by a peculiar series of the forms of organic life, that these forms characterized those periods, and that the different strata could be identified in different localities and otherwise doubtful cases by peculiar imbedded organic remains[5]." [Footnote 3: _Memoir of William Smith_, p. 142.] [Footnote 4: _Ibid._ p. 141.] [Footnote 5: The work of Smith which directly bears upon the establishment of the law of identification of strata by included organisms is published in two treatises, entitled:-- (i) _Strata identified by Organized Fossils_, 4to. (intended to comprise seven parts, of which four only were published), commenced in 1816. (ii) _A Stratigraphical System of Organized Fossils_, compiled from the original Geological Collection deposited in the British Museum. 4to. 1817.] William Smith seems to have recognised intuitively the truth of a law which was but dimly understood before his time,--the law of superposition, which may be thus stated: "of any two strata, the one which was originally the lower, is the older." This may appear self-evident but it was certainly not so. As the result of this recognition he established the second great stratigraphical law, with which his name will ever be linked, that strata are identifiable by their included organisms.