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|OriginOfSpecies - 475 Rows|
|14 - Recapitulation and Conclusion||14-02 - Recapitulation of the general and special circumstances in its favour||30||
As geology plainly proclaims that each land has undergone great physical changes, we might have expected that organic beings would have varied under nature, in the same way as they generally have varied under the changed conditions of domestication.
And if there be any variability under nature, it would be an unaccountable fact if natural selection had not come into play.
It has often been asserted, but the assertion is quite incapable of proof, that the amount of variation under nature is a strictly limited quantity.
Man, though acting on external characters alone and often capriciously, can produce within a short period a great result by adding up mere individual differences in his domestic productions; and every one admits that there are at least individual differences in species under nature.
But, besides such differences, all naturalists have admitted the existence of varieties, which they think sufficiently distinct to be worthy of record in systematic works.
No one can draw any clear distinction between individual differences and slight varieties; or between more plainly marked varieties and subspecies, and species.
Let it be observed how naturalists differ in the rank which they assign to the many representative forms in Europe and North America.
|04 - Natural Selection||04-13 - Convergence of Character||40||As far as mere inorganic conditions are concerned, it seems probable that a sufficient number of species would soon become adapted to all considerable diversities of heat, moisture, &c.; but I fully admit that the mutual relations of organic beings are more important; and as the number of species in any country goes on increasing, the organic conditions of life must become more and more complex.|
|01 - Variations Under Domestication||01-01 - Causes of Variability||20||
As far as I am able to judge, after long attending to the subject, the conditions of life appear to act in two ways,- directly on the whole organization or on certain parts alone, and indirectly by affecting the reproductive system.
With respect to the direct action, we must bear in mind that in every case, as Professor Weismann has lately insisted, and as I have incidentally shown in my work on Variation under Domestication, there are two factors: namely, the nature of the organism, and the nature of the conditions.
The former seems to be much the more important; for nearly similar variations sometimes arise under, as far as we can judge, dissimilar conditions; and, on the other hand, dissimilar variations arise under conditions which appear to be nearly uniform.
The effects on the offspring are either definite or indefinite.
They may be considered as definite when all or nearly all the offspring of individuals exposed to certain conditions during several generations are modified in the same manner.
It is extremely difficult to come to any conclusion in regard to the extent of the changes which have been thus definitely induced.
There can, however, be little doubt about many slight changes,- such as size from the amount of food, colour from the nature of the food, thickness of the skin and hair from climate, &c.
Each of the endless variations which we see in the plumage of our fowls must have had some efficient cause; and if the same cause were to act uniformly during a long series of generations on. many individuals, all probably would be modified in the same manner.
Such facts as the complex and extraordinary out-growths which variably follow from the insertion of a minute drop of poison by a gall-producing insect, show us what singular modifications might result in the case of plants from a chemical change in the nature of the sap. Indefinite variability is a much more common result of changed conditions than definite variability, and has probably played a more important part in the formation of our domestic races.
|14 - Recapitulation and Conclusion||14-02 - Recapitulation of the general and special circumstances in its favour||60||
As each species tends by its geometrical ratio of reproduction to increase inordinately in number; and as the modified descendants of each species will be enabled to increase by so much the more as they become more diversified in habits and structure, so as to be enabled to seize on many and widely different places in the economy of nature, there will be a constant tendency in natural selection to preserve the most divergent offspring of any one species.
Hence during a long-continued course of modification, the slight differences, characteristic of varieties of the same species, tend to be augmented into the greater differences characteristic of species of the same genus.
New and improved varieties will inevitably supplant and exterminate the older, less improved and intermediate varieties; and thus species are rendered to a large extent defined and distinct objects.
Dominant species belonging to the larger groups tend to give birth to new and dominant forms; so that each large group tends to become still larger, and at the same time more divergent in character.
But as all groups cannot thus succeed in increasing in size, for the world would not hold them, the more dominant groups beat the less dominant.
This tendency in the large groups to go on increasing in size and diverging in character, together with the almost inevitable contingency of much extinction, explains the arrangement of all the forms of life, in groups subordinate to groups, all within a few great classes, which we now see everywhere around us, and which has prevailed throughout all time.
This grand fact of the grouping of all organic beings seems to me utterly inexplicable on the theory of creation.
As natural selection acts solely by accumulating slight, successive, favourable variations, it can produce no great or sudden modification; it can act only by very short and slow steps.
Hence the canon of `Natura non facit saltum,' which every fresh addition to our knowledge tends to make more strictly correct, is on this theory simply intelligible.
We can plainly see why nature is prodigal in variety, though niggard in innovation.
But why this should be a law of nature if each species has been independently created, no man can explain.