And this generalization is, as now so repeatedly stated, that while the form of isolation which we know as natural selection depends for its action upon the intercrossing of all the individuals which it isolates (i.
In order to secure polytypic evolution, intercrossing between the different beneficial variants which may arise must be prevented; and there is nothing to prevent such intercrossing in the process of natural selection per se.
Therefore, it would be mere fatuity in any one to adduce free intercrossing as a "difficulty" against natural selection alone being competent to produce evolution of this kind.
Observe that one great consequence of duly recognizing the importance of intercrossing is indefinitely to raise our estimate of the part played by the principle of cross-infertility in diversifying organic nature.
These are wonderfully persistent, and have been so since historical times, intermediate characters only appearing where there has been intercrossing between different races.
We refer the reader to the section on the intercrossing of individuals (pp.
In domestication, this intercrossing may be prevented; and in this prevention lies the art of producing varieties.
These varieties would cross and blend where they met; but to this subject of intercrossing we shall soon have to return.
But as soon as the area is divided into two portions the intercrossing is stopped, and the usual result is that two closely allied races, classed as representative species, become formed.
As these causes are either constantly in action or recur annually, it is not surprising that almost all the species should be unchanged owing to the frequent intercrossing of freshly-arrived specimens.
The results of intercrossing them each way are again "hybrids" which breed true.
By the repetition of the same process, and by the occasional intercrossing of the survivors, there would be some progress, slow and fluctuating though it would be, towards the admirably coordinated structure of the giraffe.
But when we go beyond the limits of the same species, free intercrossing is barred by the law of sterility.
A discussion on the intercrossingof hermaphrodites in relation to Knight's views occurs in the Origin, Ed.
The swamping effect of intercrossingis referred to in the Origin, Ed.
In some cases, I do not doubt that the intercrossing of species, aboriginally distinct, has played an important part in the origin of our domestic productions.
The intercrossing will most affect those animals which unite for each birth, which wander much, and which do not breed at a very quick rate.
On the above principle, nurserymen always prefer getting seed from a large body of plants of the same variety, as the chance of intercrossing with other varieties is thus lessened.
In man's methodical selection, a breeder selects for some definite object, and free intercrossing will wholly stop his work.
Intercrossing plays a very important part in nature in keeping the individuals of the same species, or of the same variety, true and uniform in character.
After such primitive forms, members of different families, had become established on trees, a process of arboreal segregation or isolation would set in, and intercrossing with low-feeders would cease.
Sivert turned round and fled, as if the lightnings of retribution were at his heels, dodging in a zigzag through a maze of intercrossing streets.
By the repetition of the same process, and by the occasional intercrossing of the survivors, there would be some progress, slow and fluctuating though it would be, towards the admirably co-ordinated structure of the giraffe.
He believed that some degree of change was produced by the physical conditions of life, the intercrossing of species, and by habits causing increased use or disuse of parts.
The benefit which heterostyled dimorphic plants derive from the existence of the two forms is sufficiently obvious, namely, the intercrossing of distinct plants being thus ensured.
But the lake-dwellers usually improved the breed by intercrossing with forms derived from their own fauna.
It was an adaptable stock giving rise to many marked and vigorous varieties, from whose intercrossing something great and good could hardly fail to arise.
The recognition-marks were, I believe, not produced to prevent intercrossing, but intercrossing has been prevented because of preferential mating between individuals possessing special recognition-marks.
Darwin himself; (3) that the swamping effects of free intercrossing must always render impossible by natural selection alone any evolution of species in divergent (as distinguished from serial) lines of change.
The numerous hybrid forms now existing are the result chiefly of the intercrossing of that or other long-flowered with globose-flowered plants.
They have been raised by crossing and intercrossing the most suitable European forms with others since imported from Russia.
No advantage, as far as we can judge, was derived from intercrossing two of the grandchildren of Hero, any more than when two of the children were crossed.
Negative effects of intercrossing flowers on the same plant.
It was therefore necessary to ascertain by experiment what would be the effect of intercrossing flowers on the same plant, in comparison with fertilising them with their own pollen or crossing them with pollen from a distinct plant.
The fact of bees and of some Diptera visiting the flowers of the same species as long as they can, instead of promiscuously visiting various species, favours the intercrossing of distinct plants.
On page 179 of Darwinism he argues, most ingeniously, that the sterility of hybrids has been actually produced by natural selection to prevent the evils of the intercrossing of allied species.
Whence it inevitably follows, as a matter of simple logic, that where divergence has occurred, intercrossing and interbreeding must in some way have been lessened or prevented.
Interbreeding of Allied Species That these recognition marks prevent the intercrossing of allied species and the production of infertile hybrids appears to be pure fiction.
This is very sound criticism, and is not very materially affected by the fact that the intercrossing of varieties does not necessarily imply a blending of their characters in the offspring; for, as we have seen, some characters do not blend.
Isolation, by reducing or preventing the intercrossing which holds the individual true to the normal type of the species, tends to produce divergences.
This occasional ethnic intercrossing can be proved in greater or less degree of all island people.
Without the bar of sterility, in Mr. Romanes' opinion, free intercrossingmust render the formation of species impossible.
On the contrary, as I say in my paper, it is calculated to give this struggle a better chance than ever to develope adaptive character in the sexually isolated forms, because the swamping effects of intercrossing are diminished.
I allude to your remark about the extraordinary differences that obtain among different genera with regard to the capability of intercrossing exhibited by their constituent species.
Consequently, if intercrossingbe prevented, there is no reason why unuseful variations should not be perpetuated by heredity quite as much as useful ones when under the nursing influence of natural selection.
These changes and all others will have been kept uniform by the freeintercrossing of many individuals.
The free intercrossing of the many individuals belonging to the same species would ultimately tend to make any change of colour, thus induced, uniform in character.
The individuals of both sexes, however affected, will have been kept at each successive period nearly uniform by the free intercrossing of many individuals.
Evolution has to make head against the constant tendency of intercrossing to obliterate individual distinctions.
His explanation depends on some conception of the potentialities of conjugation and intercrossing which I confess I cannot understand.
The above list will hopefully give you a few useful examples demonstrating the appropriate usage of "intercrossing" in a variety of sentences. We hope that you will now be able to make sentences using this word. Other words: crossing; intersection; overpass; rotary; viaduct
