|
11 - Geographical Distribution
|
11-05 - Means of dispersal, by changes of climate and of the level of the land, and by occasional means
|
5
|
Sir C. Lyell and other authors have ably treated this subject.
 | | Sir Charles Lyell |
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 when its climate was different may have been a high road for migration, but now be impassable; 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 or may formerly have blended: where the sea now extends, land may at a former period have connected islands or possibly even continents together, and thus have allowed terrestrial productions to pass from one to the other.
No geologist will dispute that great mutations of level have occurred within the period of existing organisms.
Edward Forbes insisted that all the islands in the Atlantic must recently have been connected with Europe or Africa, and Europe likewise with America.
 | | Edward Forbes |
 | | island |
 | | Atlantic Ocean |
 | | europe |
 | | Africa |
 | | America |
|
|
01 - Variations Under Domestication
|
01-02 - Effects of Habit
|
10
|
Changed habits produce an inherited effect, as in the period of the flowering of plants when transported from one climate to another. With animals the increased use or disuse of parts has had a more marked influence; thus I find in the domestic duck that the bones of the wing weigh less and the bones of the leg more, in proportion to the whole skeleton, than do the same bones in the wild-duck;
 | | duck |
and this change may be safely attributed to the domestic duck flying much less, and walking more, than its wild parents.
|
|
01 - Variations Under Domestication
|
01-04 - Inheritance
|
10
|
The results of the various, unknown, or but dimly understood laws of variation are infinitely complex and diversified.
It is well worth while carefully to study the several treatises on some of our old cultivated plants, as on the hyacinth, potato, even the dahlia, &c. and it is really surprising to note the endless points of structure and constitution in which the varieties and sub-varieties differ slightly from each other.
 | | hyacinth |
 | | potato |
 | | dahlia |
The whole organisation seems to have become plastic, and departs in a slight degree from that of the parental type.
|
|
01 - Variations Under Domestication
|
01-05 -Character of Domestic Varieties
|
10
|
When we look to the hereditary varieties or races of our domestic animals and plants, and compare them with closely allied species, we generally perceive in each domestic race, as already remarked, less uniformity of character than in true species.
 
Domestic races often have a somewhat monstrous character; by which I mean, that, although differing from each other, and from other species of the same genus, in several trifling respects, they often differ in an extreme degree in some one part, both when compared one with another, and more especially when compared with the species under nature to which they are nearest allied. With these exceptions (and with that of the perfect fertility of varieties when crossed,- a subject hereafter to be discussed), domestic races of the same species differ from each other in the same manner as do the closely-allied species of the same genus in a state of nature, but the differences in most cases are less in degree.
This must be admitted as true, for the domestic races of many animals and plants have been ranked by some competent judges as the descendants of aboriginally distinct species, and by other competent judges as mere varieties.
|
