fact is to be observed in many
of the earlier geological epochs, a group progressing and reaching a
maximum of size or complexity and then dying out, or leaving at most but
few and pigmy representatives.

_Cause of Extinction of Large Animals._

Now there are several reasons for the repeated extinction of large
rather than of small animals. In the first place, animals of great bulk
require a proportionate supply of food, and any adverse change of
conditions would affect them more seriously than it would smaller
animals. In the next place, the extreme specialisation of many of these
large animals would render it less easy for them to be modified in any
new direction suited to changed conditions. Still more important,
perhaps, is the fact that very large animals always increase slowly as
compared with small ones--the elephant producing a single young one
every three years, while a rabbit may have a litter of seven or eight
young two or three times a year. Now the probability of favourable
variations will be in direct proportion to the population of the
species, and as the smaller animals are not only many hundred times more
numerous than the largest, but also increase perhaps a hundred times as
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 existence. 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 gr