touch of competent skill to restore their forms and determine their relations. Acquainted with the intimate structure of plants, knowing their living orders, and understanding how they are affected by external conditions, the fossil botanist, though dealing with a very obscure and difficult subject, can arrive at many important determinations. The evidence he has to deal with may be deficient, but it is never deceptive, if right methods are taken, and nothing assumed beyond what is apparent. And so, also, but in a more satisfactory manner, with fossil zoology. To the skilful anatomist, every bone, and tooth, and scale, and fin-spine is perfect evidence of class, or order, or family; and as we have no ground for supposing that the relations of life to external conditions were ever different from what they are at the present day, he can indicate the conditions-terrestrial, aquatic, or aërial-tropical, temperate, or boreal-under which the creatures existed with certainty, always in proportion to the perfection in which the animal has been preserved. And more knowing the great law of co-relation, which adapts every portion of the animal framework to every other portion for the performance of certain functions and the accomplishment of certain ends, he can restore the forms and indicate the habits of the individual creatures that formerly peopled our globe, and thus present a picture of its successive epochs such as the geographer attempts at the present day. He may err in his interpretations, or may allow his imagination to overrule his judgment; but the evidence remains reliable and certain as ever to those who are more competent to use it.

Besides the direct evidence of modern events, and the circumstantial evidence that refers to bygone ages, the geologist has occasionally to draw his inferences negativelyor, in other words, to conclude that certain things did not exist, or certain events did not happen, because all evidence

to the contrary is wanting. Such a course may not be altogether satisfactory—and the opponents of Geology have generally made the most of this so-called "negative evidence ;" but in many instances, in the affairs of everyday life as well as in matters of science, we have no other ground upon which to found our convictions. No discoverer has yet demonstrated the existence of great sea-serpents, mermen, or mermaids; and hence, though all parts of the ocean have not been explored, men in general disbelieve in mermaids, mermen, and sea-serpents, and this entirely on the faith of negative evidence. No hippopotamus has been seen in the rivers of America, and on the strength of this negative testimony we exclude the hippopotamus from the fauna of the New World. In the same manner, because research has hitherto failed to detect, geologists do not believe that fishes existed prior to the upper Silurian period, reptiles before the Carboniferous era, birds antecedent to the New Red Sandstone, or true mammalia earlier than the Tertiary epoch. It is true that this is negative testimony, and nothing more; but mankind must believe provisionally according to their knowledge; and surely what is universally acted upon in other matters cannot be consistently refused to geology.

Such, then, in brief and general terms, is the nature of geological evidence, and the methods in which it is employed in geological reasoning. So far as honest observation goes, there can be nothing uncertain in the evidence; so far as competent knowledge can interpret, there need be nothing unreliable in the deductions. It is upon evidence. such as this-partly direct, partly circumstantial, and partly negative that geology bases her conclusions-her conclusions of the inconceivable antiquity of the earth, the frequent changes its surface distributions have undergone, the repeated extinctions of old, and introduction of new, spe

cies of plants and animals, the gradual ascent from lower to higher forms of life, the evolution of the whole vital scheme according to some plan of progressive development, and other similar deductions, which mark a new era in human thought, as well as in human philosophy.

Concerning subjects so vast and novel, it was not to be expected that the deductions were to pass unchallenged, or the testimony to remain unquestioned; but it is expected, and it is demanded, that both should be treated with fairness, and not subjected to misrepresentation. If geologists should occasionally decline to be decisive, it is not because their evidence is uncertain, but simply because they wish it to be more complete. If they should sometimes seem to hesitate, it is not that they feel their argument is weak, but simply because, in dealing with phenomena so complex and wonderful, they shrink from being dogmatic. It is true they may occasionally doubt; but, as Humboldt observes, "physical philosophy doubts because it seeks to investigate, distinguishes between that which is certain and that which is merely probable, and strives continually to perfect theory by extending the circle of observation." Let us continue, then, to observe; let us weigh well between the certain and the probable; and let us doubt rather than arrive at conclusions where the data are scanty or in any way liable to misinterpretation. And if the great German philosopher has pronounced emphatically that true "geognosy is a science as capable of certainty as any of the physical descriptive sciences can be," we may surely accept the dictum of his gigantic and sagacious understanding, and continue to work under its encouragement, rather than be deterred by the clamour of a wilderness of unreason and prejudice. And even where the facts are too complicated or too obscure for our keenest discernment, and the truth lies far beyond the grasp of our strained and wavering com

prehensions, let us console ourselves with the remark of Dugald Stewart, that, "when we cannot trace the process by which an event has been produced, it is often of importance to be able to show how it may have been produced by natural causes; for by this means the mind is not only to a certain degree satisfied, but a check is given to that indolent philosophy which refers to a miracle whatever appearances in the natural or moral worlds it is unable to explain."

In this world of ours we see nothing save a ceaseless round of causes and effects, of means and ends, of processes and products; and we may depend upon it that these activities and results will never be understood unless we work under the inspiring belief that they were all intended to be known. The way to their comprehension may be toilsome, the timid may halt, and the stoutest may fail, but mankind may rely that satisfactory and permanent convictions of God's workings in nature will never be arrived at except through the exercise of the observing mind and the reasoning intellect. In the case of our own special science, let this be the hope that impels and the belief that determines. Its facts and evidences are scattered everywhere around and beneath us; they are graven in indelible characters on the rock-formations of the crust: let us rest not till we have read; and when we have read, let us rest not till the newer knowledge they impart has become the common property of man.

UNIFORMITY AND PROGRESSION.

In the earlier days of geology every unusual appearance on the surface or in the crust of the earth was ascribed to cataclysms and convulsions of nature-if not to cataclysms, at least to something unwonted and abnormal. Was it a clay containing huge water-worn blocks and boulders, then it was the result of some extraordinary current or oversweeping wave of translation; a deep ravine or precipitous mountain-gorge, then it arose from some internal convulsion; or were it the coal-formation, with its thick and widespread masses of transformed vegetation, then abnormal conditions of climate and atmosphere were evoked to account for the phenomena. These notions arose, no doubt, partly from a limited and imperfect knowledge of nature, but chiefly from traditionary beliefs respecting the age of the earth, which required all the appearance in the rocky crust and all its external features to have been produced within the brief space of a few thousand years. Where the phenomena were so vast, and the time so limited, the producing causes must have been correspondingly gigantic, and thus it was that the idea of cataclysms and convulsions took possession of the scientific mind. As observation was extended, and men's knowledge of nature became