mall
increase: fossils become rare: possibly sudden extermination as
Australia, but as present means very slow and many means of escape, I
shall doubt very sudden exterminations. Who can explain why some species
abound more,--why does marsh titmouse, or ring-ouzel, now little
change,--why is one sea-slug rare and another common on our coasts,--why
one species of Rhinoceros more than another,--why is <illegible> tiger of
India so rare? Curious and general sources of error, the place of an
organism is instantly filled up.

{121} On rarity and extinction see _Origin_, Ed. i. pp. 109, 319,
vi. pp. 133, 461.

We know state of earth has changed, and as earthquakes and tides go on,
the state must change,--many geologists believe a slow gradual cooling.
Now let us see in accordance with principles of [variation]
specification explained in Sect. II. how species would probably be
introduced and how such results accord with what is known.

The first fact geology proclaims is immense number of extinct forms, and
new appearances. Tertiary strata leads to belief, that forms gradually
become rare and disappear and are gradually supplied by others. We see
some forms now becoming rare and disappearing, we know of no sudden
creation: in older periods the forms _appear_ to come in suddenly, scene
shifts: but even here Devonian, Permian &c. [keep on supplying new links
in chain]--Genera and higher forms come on and disappear, in same way
leaving a species on one or more stages below that in which the form
abounded.

<GEOGRAPHICAL DISTRIBUTION.>

Sec. VI. Let us consider the absolute state of distribution of organisms of
earth's face.

Referring chiefly, but not exclusively (from difficulty of transport,
fewness, and the distinct characteristics of groups) to Mammalia; and
first considering the three or four main [regions] divisions; North
America, Europe, Asia, including greater part of E. Indian Archipelago
and Africa are intimately allied. Africa most distinct, especially most
southern parts. And the Arctic regions, which unite N. America, Asia and
Europe, only separated (if we travel one way by Behring's St.) by a
narrow strait, is most intimately allied, indeed forms but one
restricted group. Next comes S. America,--then Australia, Madagascar
(and some small islands which stand very remote from the land). Looking
at these main divisions separately, the organisms vary according to
changes in condition{122} of differe