THE AGE OF THE EARTH 73 



rock-records seem to indicate either a diminishing range of 

 variation or an increasing rate of deposition. Either it took 

 longer for the older strata to accumulate, or the plants and 

 animals which are fossilized in them changed from one 

 form into another with greater rapidity than they did in 

 later periods of geological time. Again, it might be urged 

 that as specialization increases all the openings for new 

 developments are filled up. With the present almost infinite 

 variety of forms it is almost impossible for a plant or an 

 animal to discover an effective new departure. 



