304 EVOLUTIONAL GEOLOGY. 



thick ice and moved only in obedience to the tides. The earth, frozen 

 and dark except for the red glow of her volcanoes, waited the coming 

 of the sun, and it was not till his growing splendor had banished the 

 long night that the cheerful sound of running waters was heard again 

 in our midst. Then the work of denudation and deposition seriously 

 recommenced, not to cease till the life of the sun is spent. Thus the 

 thickness of the stratified series may be a measure rather of the dura- 

 tion of sunlight than of the period Avhich has elapsed since the first 

 formation of the ocean. It ma}' have been so — we can not tell — but 

 it may be fairly urged that we know less of the origin, history, and 

 constitution of the sun than of the earth itself, and that, for aught we 

 can say to the conti'ary, the sun may have been shining on the just- 

 formed ocean as cheerfully as he shines to-day. 



TIME REQUIRED FOR THE P^VOLUTION OF THE LIVING WORLD. 



But, it will be asked, how fur does a period of twenty-six millions 

 satisfy the demands of biology? Speaking only for myself, although 

 I am aware that eminent biologists are not wanting who share this 

 opinion, I answer, amply. But it will be exclaimed, surely there are 

 " comparisons in things." liook at Egypt, where more than four 

 thousand years since the same species of man and animals lived and 

 fiourishcd as to-day. Examine the frescoes and stud}" the living pro- 

 cession of familiar forms they so faithfully portraj^, and then tell us, 

 how comes it about that from changes so slow as to be inappreciable 

 in the lapse of forty centuries you propose to build up the whole 

 organic world in the course of a mere twentj'-six millions of 3'ears? 

 To all which we might repl}^ that even changeless Eg3"pt presents us 

 with at least one change — the features of the ruling race are to-day 

 not quite the same as those of the Pharaohs. But putting this on one 

 side, the admitted constancy in some few common forms proves very 

 little, for so long as the environment remains the same natural selec- 

 tion will conserve the t3^pe, and, so far as we are able to judge, con- 

 ditions in Egypt have remained remarkably constant for a long period. 



Change the conditions, and the resulting modification of the species 

 becomes manifest enough; and in this connection it is only necessary to 

 recall the remarkable mutations observed and recorded by Professor 

 Weldon in the case of the crabs in Pl3^mouth Harbor. In response to 

 increasing turbidity of the sea water these crabs have undergone or are 

 undergoing a change in the relative dimensions of the carapace, which 

 is persistent, in one direction, and rapid enough to be determined by 

 measurements made at intervals of a few years. 



Again animals do not all change their characters at the same rate; 

 some are stable, in spite of changing conditions, and these have 

 been cited to proA^e that none of the periods we look upon as probable, 

 not twenty-five, not a hundred millions of years, scarce any period 



