PROGRESS IN THE NINETEENTH CENTURY 491 



tions. Many of the inferences have gained a certification so good and 

 so familiar that in respect to verity they take rank with seen things, 

 and we are apt to forget their origin. The successive deposition of 

 bedded rocks, the organic origin of fossils, the original horizontality 

 and continuity of folded and eroded strata: these inferences are to- 

 day accepted as if they had never been doubted; but they all were 

 once doubted, and they had to make their way against opposition. 

 Whatever order of certainty they have now acquired, they are not 

 facts of observation, but facts of inference; and, like the great body 

 of earth-science, these well-accepted facts of past time have not 

 been determined by direct seeing, but by inference on the basis of 

 seeing. We may, therefore, justly claim great progress for earth- 

 science not only in the extent and accuracy of our observations, but 

 also in the extent and accuracy of our inferences. While there is 

 yet need of more conscious recognition and more thorough training, 

 especially in the deductive processes by which many problems may 

 be solved, we may still say that among the most significant steps 

 that we have taken in the past century are those by which the neces- 

 sity and the value of theorizing have gained frank acceptance 

 among investigators and by which many of the results of theorizing 

 have gained an order of verity that compares well with that of facts 

 of mere observation. 



An illustration of this phase of our progress is to be found in two 

 definitions, each of which has a certain currency. By some writers, 

 geology is defined as the study of the earth's crust, thus emphasizing 

 the observational side of the subject; by others, geology is defined 

 as the study of the earth's history, thus giving fuller recognition 

 to the growth of inference upon observation. The second definition 

 does not lessen the essential importance of observation as the found- 

 ation of knowledge, but it accords a proper value to inferences, and 

 in this way is characteristic of what seems to me sound scientific 

 progress. The earth's crust contains the incomplete, partly con- 

 cealed, partly undecipherable records on which we are to construct 

 the science of geology: just as human monuments and writings are 

 the records on which we are to construct human history; but in 

 neither case are the records and the history identical, for the history 

 in both cases includes a great body of inferences as well as of more 

 directly recorded or observed facts. 



The wholesome appeal to observation in the search for visible facts 

 has loosened the control of supposed authority, and has given us 

 much of the freedom necessary for progress; but the assistance of 

 the trained imagination in the search for invisible facts has in a far 

 greater degree corrected the assumptions of an earlier stage of in- 

 quiry; it has even revised the dicta of philosophy and remodeled 

 the dogmas of religion. 



