XIII THE GEOLOGICAL EVIDENCES OF EVOLUTION 395 



rapidly, they are able to become quickly modified by variation 

 and natural selection in harmony with changed conditions, 

 while the large and bulky species, being unable to vary quickly 

 enough, are obliged to succumb in the struggle for exist- 

 ence. As Professor Marsh well observes : "In every vigorous 

 primitive type which was destined to survive many geological 

 changes, there seems to have been a tendency to throw off 

 lateral branches, which became highly specialised and soon 

 died out, because they were unable to adapt themselves to new 

 conditions." And he goes on to show how the whole narrow 

 path of the persistent Suilline type, throughout the entire 

 series of the American tertiaries, is strewed with the remains of 

 such ambitious offshoots, many of them attaining the size of 

 a rhinoceros ; " while the typical pig, with an obstinacy never 

 lost, has held on in spite of catastrophes and evolution, and still 

 lives in America to-day." 



Indications of General Progression in Plants and Animals. 



One of the most powerful arguments formerly adduced 

 against evolution was, that geology afforded no evidence of 

 the gradual development of organic forms, but that whole 

 tribes and classes appeared suddenly at definite epochs, and 

 often in great variety and exhibiting a very perfect organisa- 

 tion. The mammalia, for example, were long thought to have 

 first appeared in Tertiary times, where they are represented in 

 some of the earlier de}X)sits by all the great divisions of the 

 class fully developed — carnivora, rodents, insectivora, mar- 

 supials, and even the perissodactyle and artiodactyle divisions of 

 the ungulata — as clearly defined as at the present day. The 

 discovery in 1818 of a single lower jaw in the Stonesfield 

 Slate of Oxfordshire hardly threw doubt on the generalisation, 

 since either its mammalian character was denied, or the 

 geological position of the strata, in which it was found, was 

 held to have been erroneously determined. But since then, at 

 intervals of many years, other remains of mammalia have been 

 discovered in the Secondary strata, ranging from the Upper 

 Oolite to the Upper Trias both in Europe and the United 

 States, and one even (Tritylodon) in the Trias of South Africa. 

 All these are either marsupials, or of some still lower type of 

 mammalia ; but they consist of many distinct forms classed in 



