Such are the wonderful contrivance, adaptation, and design which the material world every where exhibits. But the geologist carries us back through periods of immense antiquity, and digs out from the deep strata evidences of other systems of organic life, which have flourished and passed away; other economies, which have existed on the globe anterior to the present. And how was it with these? Had they any relation to the existing system? Were they governed by different laws, or are they all but parts of one great and harmonious system, embracing the whole of the earth's past duration? We could not decide these questions beforehand; but geology brings to light unequivocal evidence that the latter supposition is the true one; that is, in the language of the poet,-- "All are but parts of one stupendous whole, Whose body nature is, and God the soul." To present the evidence of this conclusion will be my object in this lecture. _In the first place, the laws of chemistry and crystallography, electricity and magnetism, have ever been the same in all past conditions of the earth._ Chemistry has attained to such a degree of perfection that the analyst can now determine the composition of the various vegetable, animal, and mineral substances which he meets, with an extreme degree of accuracy. In many instances, he can do this in two ways. He can always separate the elements which exist in a compound, and ascertain their relative quantity; and this is called _analysis_. And sometimes he can take those elements and cause them to unite, so as to form a particular compound; and this is called _synthesis_. By these methods he has ascertained that, amid the vast variety of substances in nature, there are only about sixty-four which cannot be reduced to a more simple form, and are therefore called _elements_, or simple substances. Now, the chemist finds that, when these elements unite to form compounds, certain fixed laws are invariably followed. They combine in definite quantities, which are always the same, or some multiple of the same weight; so that each element has its peculiar and invariable combining weight; and it cannot be made to combine in any other proportion. You may mix two or more elements together in any proportion, but it is only a certain definite quantity of each that will combine, while the rest will remain in excess. Hence the same compound substance, from whatever part of the world it comes, or under however diverse circumstances produced, consists of the same ingredients in the same proportion. These laws are followed with mathematical precision, and we have reason to believe that the same compound substance, produced in different parts of the world, never differs in its composition by the smallest conceivable particle. Indeed, with the exception of the planetary motions and crystallography, chemical combination is the most perfect example of practical mathematics to be found in nature.