ANIMALS OF THE PAST 341 



ment of the different phyla and to specialization within 

 particular groups Protozoa must have preceded Metazoa; 

 crustaceans existed before insects. Fishes, amphibians, 

 reptiles, birds and mammals have arisen since Pre-cambrian 

 times and the order of their appearance agrees roughly 

 with their degree of specialization. What has been said 

 of animals applies equally well to plants. There has been 

 a succession of algae, mosses, ferns, conifers, and finally the 

 seed-plants of today. 



There have been great changes; the types of animals 

 living today are few compared with those which have passed 

 away forever. The animal kindgom past and present may 

 be compared to a cotton-wood tree which is being covered 

 by a moving dune; the living twigs are but a small part of 

 the portions now covered by the shifting sands of time or 

 dune. 



