84 Evolution 



partly from the anxieties of their frail existence, an 

 organ was developed in them that the huge reptiles had 

 not needed to cultivate. This was the brain. The 

 ninety-ton Brontosaur appears to have had the brain 

 of a nine-pound human infant, and all his cousins had 

 the slenderest amount of brain in their formidable skulls. 

 The race now began to depend more on intelligence ; 

 possibly the new development of maternal feeling, in the 

 greater care of the young, increased the accent on mind. 

 At all events the new inhabitant of the planet spread 

 rapidly over its surface. Before the end of the Jurassic 

 we find thirty-three genera of mammals, ranging from 

 Europe to America across the "lost Atlantis." All 

 belonged to the lowest classes of the mammal world, the 

 Monotremes and Marsupials, in which the uterine 

 arrangements are of a primitive order. Some authori- 

 ties think that the whole of them were Marsupials — 

 leaping animals of the opossum type, bearing their 

 young in pouches or folds of the skin. Whether these 

 Marsupials were developed from the Monotreme, as 

 some think, or both had a common reptile ancestor, we 

 may leave open. However that may be, we know that 

 these Marsupials gradually overran the earth, penetrating 

 to South America on the one hand, and Australia on the 

 other. Australia seems to have been cut off from about 

 the end of the Secondary epoch, and this accounts for 

 the fact of its mammals remaining at the marsupial 

 stage. 



From one or other of these early mammals all the 

 varied forms we are familiar with have been evolved. 

 Once more we must refrain from an attempt to trace 

 the lines of their descent, on account of the magnitude 

 and conjectural nature of the task. The fossil remains 

 we have from the beginning of the Tertiary epoch are in 

 complete accord with the theory of evolution. At first 



