ABRUPT AND GRADUAL IMPRESSIONS. 483 



great certainty and accuracy, the number of years which have been oc- 

 cupied in the formation of the trunk. The circular lines upon the cross 

 section, well known to every forester, are thence called annual rings. 

 When, fortified with the knowledge of this fact, we compare with each 

 other the trunks of the conifers which we obtain from the various epochs 

 of formation, we find that the oldest remains exhibit no trace whatever 

 of annual rings, but, in the course of time, they become continually more 

 defined, so that lastly, in the most recent formations for instance, in the 

 upper brown coal they appear marked just as distinctly as in the trees 

 now living in the same localities." 



In speaking of artificial changes impressed by culture upon domestic 

 plants which have been converted from varieties into sub- Difference be- 

 species, the importance of the element of time was insisted * w een abrupt 



T i -ti 1*11 an( l gradual 



upon. In the same manner, in the changes which have oc- impressions on 

 curred during geological periods, the successive replacement P lants - 

 of one class of vegetable forms by another, that element again obtrudes 

 itself upon our notice. If a few years serve to establish such minor 

 changes as the perpetuation of varieties into sub-species, what should be 

 expected from the enduring influence of innumerable centuries ? More- 

 over, in these artificial results there is a necessary abruptness in the appli- 

 cation of the disturbance, which can not but exert an unfavorable influ- 

 ence. No time is afforded to the organism to suit itself gradually to the 

 force exerted upon it, none for acclimating itself to the external variation. 

 It must either yield at once or perish. But how different as respects 

 the method of application in the case of the organic series ! If it be de- 

 cline of temperature that we consider, how shall we enumerate the suc- 

 cessive centuries that must have elapsed as the descent was made from 

 degree to degree ? In these later times, as is admitted on all hands, the 

 mean temperature of the surface could not decline the tenth part of a 

 Fahrenheit degree in the lapse of 10,000 years. Yet the interval has 

 transpired during which there has been a gradual descent from those 

 high thermometric points at which the existence of organic life was bare- 

 ly possible, and, in truth, through a far greater range than that. It sig- 

 nifies nothing that this descent might have been more rapid the higher 

 the degree ; in any case, it implies a prodigious interval of time. Or, if 

 we consider variations in the light of the sun, either because of his being 

 a variable star, or because of the gradual clearing up and improving 

 transparency of the atmosphere, we are brought again to the same re- 

 sult long periods of time ; for, though there may be among the fixed 

 stars some whose periods of variation, as respects brilliancy, are short, 

 included perhaps within a few days, or even hours, if we had no better 

 evidence, history assures that our sun is not one of that quickly-varying 

 group. Or, again, if we consider the changes which have indisputably 



