| → OMIT 1869 1872 |
| will not have been due, 1859 1860 1861 1866 |
|
| → will not be due to 1869 1872 |
| to 1859 1860 1861 1866 |
|
| → facts. Change of climate must have had a powerful influence on migration. 1872 |
| facts. 1859 1860 1861 1866 1869 |
|
| → A region now impassable to certain organisms from the nature of 1872 |
| Change of climate must have had a powerful influence on migration: a region when 1859 1860 1861 1866 |
| Change of climate must have had a powerful influence on migration; an impassable region when 1869 |
|
| → might 1872 |
| was different may 1859 1860 1861 1866 |
| was different from what it now is, may 1869 |
|
| → when the climate was different. 1872 |
| but now be impassable; 1859 1860 1861 1866 |
| OMIT 1869 |
|
| → have been recently 1869 1872 |
| recently have been 1859 1860 1861 1866 |
|
| → in the level of the land or sea; 1869 1872 |
| of level in our continents; 1859 1860 1861 1866 |
|
| → of our continents, as 1869 1872 |
| as 1859 1860 1861 1866 |
|
|
modification
→OMIT
at each
→will not be due to
descent from a single parent. To illustrate what I mean: our English
differ
from the horses of every other breed; but they do not
their
and
to descent from any single pair, but to continued care in
and training
individuals during
|
|
| Before discussing the three classes of facts, which I have selected as presenting the greatest amount of difficulty on the theory of
centres of
I must say a few words on the means of dispersal. |
|
|
| Sir C. Lyell and other authors have ably treated this subject. I can give here only the briefest abstract of the more important
→facts. Change of climate must have had a powerful influence on migration.
→A region now impassable to certain organisms from the nature of
its
→might
have been a high road for
→when the climate was different.
I shall, however, presently have to discuss this branch of the subject in some detail. Changes of level in the land must also have been highly influential: a narrow isthmus now separates two marine faunas; submerge it, or let it formerly have been submerged, and the two faunas will now blend
may formerly have
the sea now extends, land may at
former
have connected islands or possibly even continents together, and thus have allowed terrestrial productions to pass from one to the other. No geologist
that great mutations of
have occurred within the period of existing organisms. Edward Forbes insisted that all the islands in the Atlantic must
→have been recently
connected with Europe or Africa, and Europe likewise with America. Other authors have thus hypothetically bridged over every ocean, and
united almost every island to some mainland. If indeed the arguments used by Forbes are to be trusted, it must be admitted that scarcely a single island exists which has not recently been united to some continent. This view cuts the Gordian knot of the dispersal of the same species to the most distant points, and removes many a
but to the best of my judgment we are not authorized in admitting such enormous geographical changes within the period of existing species. It seems to me that we have abundant evidence of great oscillations
→in the level of the land or sea;
but not of such vast changes in
position and
→of our continents, as
to have united them within the recent period to each other and to the several intervening oceanic islands. I freely admit the former existence of many islands, now buried beneath the sea, which may have served as
|
