THE REIGN OF MAMMALS. 223 



seventy feet, and are undoubtedly the first of the whales in 

 rank as well as in time. This is perhaps one of the most 

 difficult facts to be explained on the theory of evolution. 

 Allied to the whales is the small and peculiar group of the 

 Sea-cows or Diigongf (Sirenians). These creatures, highly 

 specialised and very distinct from all others, appear in the 

 Early Tertiary in forms very similar to those which now exist, 

 and probably in much more numerous species, and they pursue 



Fig. i8o.-Tooth of Eocene Whale {Ze7,glodon cetioides). One-half natural size. 



the even tei. . of their way down to modern times without 

 perceptible elevation or degradation. " We have questioned," 

 says Gaudry, when speakingof the Tertiary Cetaceans, " these 

 strange and gigantic sovereigns of the Tertiary oceans as to 

 their progenitors— they leave us without reply." Their silence 

 is the more significant as one can scarcely suppose t" ?se 

 animals to have been nurtured in anyhmited or secluded space 

 in the early stages of their development. The true Seals, 



