Origin of Species Variorum

→ must have been 1859 1860 1861

would have been 1866

would be 1869

will have been 1872

→ took 1859 1860 1861 1866

must have taken 1869 1872

→ become 1859 1860 1861 1869 1872

been liable to become 1866

→ the two Worlds. 1859 1860 1861

Europe and N. America. 1866 1869 1872

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

highly

most

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

whole Atlantic

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 →must have been completely cut off from each other. This separation, as far as the more temperate productions are concerned, →took place long ages ago.

And as

the plants and animals migrated southward, they

would have

would

will have

→become mingled in the one great region with the native American productions, and

would have

have

had to compete with them; and in the other great region, with those of the Old World. Consequently we have here everything favourable for much modification,— for far more modification than with the Alpine productions, left isolated, within a much more recent period, on the several mountain-ranges and on the arctic lands of →the two Worlds. Hence it has come, that when we compare