THE FUTURE OF THE LIVING WORLD. Ijg 



subsequent development has been only elaboration 

 and not the production of new types. 



Now we are not at liberty to assume an indefinite 

 amount of time prior to the Silurian. Of course it is 

 impossible to say just how long a time elapsed 

 between the origin of life and the beginning of the 

 Silurian, but it seems hardly possible that it could 

 have equalled the time that has elapsed since then. 

 But, upon evolutionary theories, the animal kingdom 

 must have developed during that period from the 

 lowest unicellular condition to the complex and 

 diversified fauna of the Silurian. When we consider, 

 therefore, that during this time all of the important 

 groups of the animal kingdom arose (with perhaps 

 the exception of the vertebrates) and that none have 

 arisen since that time, it becomes quite evident that 

 evolution must have progressed with greater rapidity 

 at that time than it has since. This conclusion is 

 no new one, for many naturalists have seen the 

 necessity of making some such assumption. It will, 

 indeed, be generally acknowledged that evolution in 

 the earliest ages was more rapid than at present. 



Here, then, we see another point of likeness in the 

 comparison of the living world with the life of the 

 individual. An individual when it begins life grows 

 most rapidly, but from the very moment of its birth 

 the rapidity of growth lessens until a stationary 

 condition is reached at maturity. So it seems in 

 the longer history of world life. Its growth was 

 most rapid at the birth of life and has been decreas- 

 ing since that time, not with regularity, perhaps, 

 being frequently interrupted by periods of more 



