Vietnam or Thailand ? Vote for the TOP Country of the Week !
Updated: June 1, 2025
If it could be shown that our domestic varieties manifested a strong tendency to reversion that is, to lose their acquired characters, while kept under the same conditions and while kept in a considerable body, so that free intercrossing might check, by blending together, any slight deviations in their structure, in such case, I grant that we could deduce nothing from domestic varieties in regard to species.
These facilities of distribution tend to assimilate the productions of adjacent lands in two ways: first, by direct mutual interchange of species; and secondly, by repeated immigrations of fresh individuals of a species common to other islands, which by intercrossing, tend to obliterate the changes of form and colour, which differences of conditions might otherwise produce.
Even with animals which unite for each birth, and which do not propagate rapidly, we must not assume that free intercrossing would always eliminate the effects of natural selection; for I can bring forward a considerable body of facts showing that within the same area two varieties of the same animal may long remain distinct, from haunting different stations, from breeding at slightly different seasons, or from the individuals of each variety preferring to pair together.
Independently of fusion from intercrossing, the complete absence, in a well-investigated region, of varieties linking together any two closely- allied forms, is probably the most important of all the criterions of their specific distinctness; and this is a somewhat different consideration from mere constancy of character, for two forms may be highly variable and yet not yield intermediate varieties.
Finally, considering all the ascertained facts on the intercrossing of plants and animals, it may be concluded that some degree of sterility, both in first crosses and in hybrids, is an extremely general result; but that it cannot, under our present state of knowledge, be considered as absolutely universal.
Consequently the whole body of males inhabiting the same country would tend from the effects of constant intercrossing to become modified almost uniformly, but sometimes a little more in one character and sometimes in another, though at an extremely slow rate; all ultimately being thus rendered more attractive to the females.
Natural Selection its power compared with man's selection its power on characters of trifling importance its power at all ages and on both sexes Sexual Selection On the generality of intercrosses between individuals of the same species Circumstances favourable and unfavourable to the results of Natural Selection, namely, intercrossing, isolation, number of individuals Slow action Extinction caused by Natural Selection Divergence of Character, related to the diversity of inhabitants of any small area and to naturalisation Action of Natural Selection, through Divergence of Character and Extinction, on the descendants from a common parent Explains the Grouping of all organic beings Advance in organisation Low forms preserved Convergence of character Indefinite multiplication of species Summary.
Romanes suggests," continues the Times, "is that at a certain stage of development of varieties in a state of nature a change takes place in their reproductive systems, rendering those which differ in some particulars mutually infertile, and thus the formation of new permanent species takes place without the swamping effect of free intercrossing. . . . How his theory can be properly termed one of selection he fails to make clear.
But we shall see in the sixth chapter that intermediate varieties, inhabiting intermediate districts, will in the long run generally be supplanted by one of the adjoining varieties. Intercrossing will chiefly affect those animals which unite for each birth and wander much, and which do not breed at a very quick rate.
Both sexes have perhaps been directly acted on by the conditions of life to which they have long been exposed: but the females from not being otherwise much modified, will best exhibit any such effects. These changes and all others will have been kept uniform by the free intercrossing of many individuals.
Word Of The Day
Others Looking