THE DEVELOPMENT OF THE ANIMAL WORLD 85 



we have very vague and generalised forms, with sufficient 

 traces of their marsupial ancestry, yet pointing clearly 

 enough to the future horse, fox, pig, etc. The great 

 improvement of the plant world during and after the 

 Cretaceous (see last chapter), and especially the 

 evolution of grasses, leads to a wide extension and 

 variety of the vegetarian mammals, and this in turn 

 favours the carnivores. The struggle intensifies, with 

 the usual effect of differentiation. In some the quality 

 of speed is selected. The foot modifies into a hoof, and 

 the horse, deer, etc., slowly appear in the successive 

 strata. As is now well known, we can trace the horse 

 back, and watch the disappearance of the missing toes, 

 as far as the four-toed Orohippus of the Eocene period. 

 With more or less success the lines of the other 

 mammals are traced back to the same period. Early 

 types of deer and antelopes (without horns), squirrels, 

 hedgehogs, bats, and lemurs, are found in the Eocene. 

 Some animals (Tillodonts) unite the characters of 

 ungulates, rodents, and carnivores: others (Deinocerata), 

 with heavy-horned skulls, remind us at once of the 

 elephant and the rhinoceros. The lower temperature of 

 the earth has now paralysed the reptiles, and opened a 

 broad field for the warm-blooded animals. 



With the Miocene we get a clearer development of 

 the cat, bear, elephant (mastodon), rhinoceros, hippo- 

 potamus, lion, dolphin, beaver, otter, and other mammals. 

 Still all are in ancestral stages, and far removed from 

 the animals to which we now give those names. The 

 Pliocene strata carry the story of their evolution a step 

 further, and the gazelle, antelope, deer, giraffe, horse, ox, 

 wild cat, bear, hyena, wolf, fox, glutton, seal, pig, musk- 

 sheep, mole, elephant, etc., stand out with great distinct- 

 ness on the closing scenes of the Tertiary epoch. There 

 are still the million years or so of the Quaternary epoch 



