788 



SCIENCE. 



[N. S. Vol XII. No. 308. 



say to the contrary, the sun may have been 

 shining on the just-formed ocean as cheer- 

 fully as he shines to-day. 



TIME KEQtriEBD FOE THE BVOLTITION OF THE 

 LIVING WOELD. 



But, it will be asked, how far 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.' Look at Egypt, where more 

 than 4,000 years since the same species of 

 man and animals lived and flourished as 

 to-day. Examine the frescoes and study 

 the living procession of familiar forms they 

 so faithfully portray, 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 

 twenty-six millions of years ? To all which 

 we might reply that even changeless Egypt 

 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 environ- 

 ment remains the same natural selection 

 will conserve the type, and, so far as we are 

 able to judge, conditions in Egypt have 

 remained remarkably constant for a long 

 period. 



Change the conditions, and the resulting 

 modification of the species becomes mani- 

 fest enough ; and in this connection it is 

 only necessary to recall the remarkable 

 mutations observed and recorded by Pro- 

 fessor Weldon in the case of the crabs in 

 Plymouth Harbor. In response to increas- 

 ing 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 measure- 

 ments 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 prove that none of 

 the periods we look upon as probable, not 

 twenty- five, not a hundred millions of years, 

 scarce any period short of eternity, is suffi- 

 cient to account for the evolution of the 

 living world. If the little tongue- shell, 

 Lingula, has endured with next to no per- 

 ceptible change from the Cambrian down 

 to the present day, how long, it is some- 

 times inquired, would it require for the 

 evolution of the rest of the animal king- 

 dom ? The reply is simple : the cases are 

 dissimilar, and the same record which as- 

 sures us of the persistency of the Lingula 

 tells us in language equally emphatic of the 

 course of evolution which has led from the 

 lower organisms upwards to man. In re- 

 cent and Pleistocene deposits the relics of 

 man are plentiful : in the latest Pliocene 

 they have disappeared, and we encounter 

 the remarkable form Pithecanthrojms ; as we 

 descend into the Tertiary systems the 

 higher mammals are met with, alwaj^s sink- 

 ing lower and lower in the scale of organi- 

 zation as they occur deeper in the series, 

 till in the Mesozoic deposits they have en- 

 tirely disappeared, and their place is taken 

 by the lower mammals, a feeble folk, ofifer- 

 ing little promise of the future they were 

 to inherit. Still lower, and even these are 

 gone ; and in the Permian we encounter 

 reptiles and the ancestors of reptiles, prob- 

 ably ancestors of mammals too ; then into 

 the Carboniferous, where we find amphib- 

 ians, but no true reptiles ; and next into 

 the Devcmian, where fish predominate, after 

 making their earliest appearance at the 

 close of the Silurian times ; thence down- 

 wards, and the vertebrata are no more 

 found — we trace the evolution of the in- 

 vertebrata alone. Thus the orderly proces- 



