



GEOLOGY, 



ON THE CONDITIONS OF GEOLOGICAL TIME. 



THE successive modifications which the views of physical geologists have 

 undergone since the infancy of their science, with regard to the amount and 

 the nature of the changes which the crust of the globe has suffered, have 

 all tended towards the establishment of the belief, that throughout that vast 

 scries of ages which was occupied by the deposition of the stratified rocks, 

 and which may be called " geological time " (to distinguish it from the " his- 

 torical time " which followed, and the " pre-geological time" which preceded 

 it), the intensity and character of the physical forces which have been in 

 operation have varied within but narrow limits; so that, even in Silurian 

 or Cambrian epochs, the aspect of physical nature must have been much 

 what it is now. This view of the condition of the earth, so far as geological 

 time is concerned, is, however, perfectly consistent Avith the notion of a to- 

 tally different state of things in antecedent epochs, and the strongest advo- 

 cate of such " physical uniformity/' during the time of which we have a 

 record, might, with perfect consistency, hold the so-called " nebular hypoth- 

 esis," or any other view involving the conception of a long series of states 

 very different from that which we now know, and whose succession occu- 

 pied pre-geological time. The doctrine of physical uniformity, and that of 

 physical progression, are therefore perfectly consistent, if we regard geolog- 

 ical time as having the same relation to pre-geological time as historical 

 time has to it. The accepted doctrines of palaeontology are by no means in 

 harmony with these tendencies of physical geology. It is generally believed 

 that there is a vast contrast between the ancient and the modern organic 

 worlds it is incessantly assumed that we are acquainted with the begin- 

 ning of life, and with the original manifestation of each of its typical forms ; 

 nor does the fact that the discoveries of every year oblige the holders of 

 these views to change their ground, appear sensibly to affect the tenacity of 

 their adhesion. Without at all denying the considerable positive differences 

 which really exist between the ancient and the modern forms of life, and 

 leaving the negative ones to be met by the other lines of argument, an im- 

 partial examination of the facts revealed by palaeontology seems to show 

 that these differences and contrasts have been greatly exaggerated. Thus, of 

 some two hundred known orders of plants, not one is exclusively fossil. 

 Among animals, there is not a single totally extinct class; and of the or- 

 ders, not more than seven per cent., at the outside, are unrepresented in the 

 existing creation. Again, certain well-marked forms of living beings have 



