PRINCIPLES AND METHODS OF PALAEONTOLOGY. 387 



existing manifestations of the vital forces, so we must not be too sure 

 that corresponding departures from the usuaL order of the physical 

 ^yorld have not occurred in past times. 



The same analogies which demand this caution, however, fully justify 

 us in concluding that throughout all geological time the great ))h3sical 

 forces have obeyed similar laws. The gravitation of matter, its hard- 

 ness, the effects of heat and of chemical affinity upon it, have been the 

 same, we have every reason to believe, from the Cambrian age to the 

 present, and, as a consequence, it cannot be doubted that the vital 

 actions of the trilobites were governed by the same physiological laws 

 as those by which we now live and move and have our being. For, 

 leaving the lihenomena of consciousness out of the question, physiology 

 is but an application of j^hysics and chemistry. 



5. Xow, just as the restorationsoftlie palaeontologist imply his confi- 

 dence in the uniformity of the great laws of morphology throughout all 

 time, so the chronology of geology, the basis of the whole science, rests 

 upon a like assumption with reg(ird to the general uniformity of the 

 laws of physics and chemistry. It would be ridiculous to argue from 

 the superposition of ancient beds, unless we assumed that their con- 

 stituent particles gravitated in the same way then as now ; the identity 

 of mineral character of two beds could prove nothing, without the as- 

 sumption that the laws governing chemical changes have always been 

 the same ; and, in like manner, we can reason on the general habits of 

 ancient living beings only on the assumption that the great laws of 

 physiology were the same then as now. No half measures will avail ; 

 we must be prepared either to assume the general uniformity of ancient 

 and modern action, or we must give up the i)roblem, for no other hy- 

 pothesis affords the least criterion of truth, or the slightest check upon 

 the play of the imagination. But if we may argue from like effects to 

 like causes, then geological chronology is as much a matter of science, 

 and capable of being tested as thoroughly, as any other case of succes- 

 sion. 



The arguments on which these chronological considerations are 

 founded are simple and intelligible enough. It has been already proved 

 that, in the present state of things, the lowest of any series of beds which 

 have been deposited from water is of necessity the oldest. If, then, 

 the great majority of the ancient strata have also been deposited from 

 water, if they are nothing but the hardened muddy beds of ancient seas 

 and lakes, (a fact of which there is abundant evidence,) then the same 

 law necessarily applies to them — the lowest stratum is the oldest, and 

 the superjacent beds have all been deposited during a subsequent period. 

 The argument applies with equal force to the whole crust of the earth, 

 and if we could tell how much time was required for the formation of 

 each bed, we should, by adding all the periods together, arrive at the 

 smallest possible interval which can have elapsed since the deposition 

 of the oldest bed. We have no data sufficient to enable us to say, with 

 any approximation to accurac}*, how long it takes to deposit sufficient 

 mud or sand to form, when hardened, a layer of rock two feet thick ; 

 but we are quite safe in .saying that neither lake nor sea ever deposited 

 that amount upon its bed in the course of a year.* Kow the total 

 measured thickness of ancient strata, deposited either from freslt or 

 salt water, is not less than 00,000 feet, (or about twelve miles,) so that, 



* Exceptional ileiJosits, as, for instance, by earthquake floods, are here left out of 

 consideration, as they can have liad but little inllueuce on the sum total of the aqueous 

 formations. The total thickness of the latter here a.ssumed is midway between the 

 estimates of Professor Philliiis and Sir C. Lyell. 



