98 EVOLUTION OF TO-DAY. 



species. The estimates of the age of the world, 

 made from geological researches of late years, are 

 smaller than those of twenty years ago, and fall 

 readily within the estimate of Sir Wm. Thomson. 

 The only difficulty lies in Darwin's three hundred 

 millions of years. Now this estimate of Darwin is 

 nothing more than a guess. We know almost 

 nothing concerning the rate of modification of 

 species to-day, not to speak of our absolute ignor- 

 ance of what it may have been in the past. Sir 

 Wm. Thomson insists that the physical changes 

 were more rapid in early times than they are now, 

 and this would necessitate a proportionate rapidity 

 in the changes of the organic world. Moreover, 

 modern embryology, as we shall see . in the next 

 chapter, seems to indicate that the history of 

 animals has not been so long and circuitous as was 

 once supposed. Instead of passing through a long 

 series of great changes, all of the large groups of 

 animals, even the vertebrates, have had a direct sim- 

 ple history from the simplest multicellular animal. 

 This early ancestral animal branched off directly 

 into several directions, and thus the time necessary 

 for the development of a diverse fauna like that of 

 the Silurian age is less than could formerly be sup- 

 posed. Again, the time since the Silurian age has 

 been sufficient to develop the group of vertebrates 

 from its simplest form to its present condition, and 

 an equal amount of time, and probably a much 

 shorter period, before the Silurian age, would be 

 amply sufficient to account for the primordial 

 fauna. The duration of this period in years is of 



