Origin of Species Variorum

↑ 2 blocks not present in 1866 1869 1872; present in 1859 1860 1861

I believe the above difficulty may be surmounted by looking to still earlier changes of climate of an opposite nature. We have good reason to believe that during the newer Pliocene period, before the Glacial epoch, and whilst the majority of the inhabitants of the world were specifically the same as now, the climate was warmer than at the present day.

→ which now live under latitude 60°, 1872

now living under the climate of latitude 60°, 1859 1860 1861

which now live under latitude 60°, 1866

which now live under latitude 60°, lived 1869

→ lived during the Pliocene period 1872

during the Pliocene period lived 1859 1860 1861 1866

during the Pliocene period 1869

→ terrestrial globe, we see 1869 1872

globe, we shall see that 1859 1860 1861

terrestrial globe, we see that 1866

→ under a more favourable climate 1869 1872

for intermigration under a more favourable climate, 1859 1860 1861

under a more favourable climate for intermigration, 1866

→ for intermigration, will account for the supposed uniformity of 1869 1872

I attribute the necessary amount of uniformity in 1859 1860 1861

I attribute a considerable degree of uniformity in 1866

→ great 1872

large, but partial 1859 1860 1861 1866 1869

→ will have been 1872

must have been 1859 1860 1861

would have been 1866

would be 1869

The explanation, I believe, lies in the nature of the climate before the commencement of the Glacial period.

During

this, the newer Pliocene period,

when

....

the majority of the inhabitants of the world were specifically the same as now,

and we

have good reason to believe that the climate was warmer than at the present day. ↑ Hence we may suppose that the organisms →which now live under latitude 60°, →lived during the Pliocene period

further

father

farther

north under the Polar Circle, in latitude

66°-67°;

and that the

strictly

present

arctic productions then lived on the broken land still nearer to the pole.

Now

Now,

if we look at a →terrestrial globe, we see under the Polar Circle

there

that there

is almost continuous land from western Europe, through Siberia, to eastern America. And

....

this continuity of the circumpolar land,

and to

with

the consequent freedom →under a more favourable climate →for intermigration, will account for the supposed uniformity of the sub-arctic and

northern

....

temperate productions of the Old and New Worlds, at a period anterior to the Glacial epoch.

Believing, from reasons before alluded to, that our continents have long remained in nearly the same relative position, though subjected to →great oscillations of level, I am strongly inclined to extend the above view, and to infer that during some

earlier

still earlier

and still warmer period, such as the older Pliocene period, a large number of the same plants and animals inhabited the almost continuous circumpolar land; and that these plants and animals, both in the Old and New Worlds, began slowly to migrate southwards as the climate became less warm, long before the

com-

....

mencement

commencement

of the Glacial period. We now see, as I believe, their descendants, mostly in a modified condition, in the central parts of Europe and the United States. On this view we can understand the

relationship,

relationship

with very little identity, between the productions of North America and Europe,— a relationship which is

most

highly

remarkable, considering the distance of the two areas, and their separation by the

Atlantic

whole Atlantic

Ocean. We can further understand the singular fact remarked on by several

observers,

observers

that the productions of Europe and America during the

latter

later

tertiary stages were more closely related to each other than they are at the present time; for during these warmer periods the northern parts of the Old and New Worlds will have been almost continuously united by land, serving as a bridge, since rendered impassable by cold, for the

inter-migration

intermigration

of their inhabitants.

During the slowly decreasing warmth of the Pliocene period, as soon as the species in common, which

inha- bited

inhabited

the New and Old Worlds,

had migrated

migrated

south of the Polar Circle, they →will have been