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 deposits 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 
