M Database Inspector (cheetah)
|Not logged in. Login|
|OriginOfSpecies - 475 Rows|
|02 - Variations Under Nature||02-06 - Many of the Species included within the Larger Genera resemble Varieties in being very closely, but unequally, related to each other, and in having restricted ranges||10||
There are other relations between the species of large genera and their recorded varieties which deserve notice.
We have seen that there is no infallible criterion by which to distinguish species and well-marked varieties; and when intermediate links have not been found between doubtful forms, naturalists are compelled to come to a determination by the amount of difference between them, judging by analogy whether or not the amount suffices to raise one or both to the rank of species.
Hence the amount of difference is one very important criterion in settling whether two forms should be ranked as species or varieties.
Now Fries has remarked in regard to plants, and Westwood in regard to insects, that in large genera the amount of difference between the species is often exceedingly small.
I have endeavoured to test this numerically by averages, and, as far as my imperfect results go, they confirm the view.
I have also consulted some sagacious and experienced observers, and, after deliberation, they concur in this view.
In this respect, therefore, the species of the larger genera resemble varieties, more than do the species of the smaller genera.
Or the case may be put in another way, and it maybe said, that in the larger genera, in which a number of varieties or incipient species greater than the average are now manufacturing, many of the species already manufactured still to a certain extent resemble varieties, for they differ from each other by less than the usual amount of difference.
Moreover, the species of the larger genera are related to each other, in the same manner as the varieties of any one species are related to each other. No naturalist pretends that all the species of a genus are equally distinct from each other; they may generally be divided into sub-genera, or sections, or lesser groups.
As Fries has well remarked, little groups of species are generally clustered like satellites around other species.
|02 - Variations Under Nature||02-07 - Summary||10||
Finally, varieties cannot be distinguished from species,- except, first, by the discovery of intermediate linking forms; and, secondly, by a certain indefinite amount of difference between them; for two forms, if differing very little, are generally ranked as varieties, notwithstanding that they cannot be closely connected; but the amount of difference considered necessary to give to any two forms the rank of species cannot be defined.
In genera having more than the average number of species in any country, the species of these genera have more than the average number of varieties.
In large genera the species are apt to be closely, but unequally, allied together, forming little clusters round other species. Species very closely allied to other species apparently have restricted ranges.
|03 - Struggle for Existence||03-02 - The Term, Struggle for Existence, used in a large sense||10||
We will now discuss in a little more detail the struggle for existence. In my future work this subject will be treated, as it well deserves, at greater length.
The elder De Candolle and Lyell have largely and philosophically shown that all organic beings are exposed to severe competition.
In regard to plants, no one has treate this subject with more spirit and ability than W. Herbert, Dean of Manchester, evidently the result of his great horticultural knowledge.
Nothing is easier than to admit in words the truth of the universal struggle for life, or more difficult- at least I have found it so- than constantly to bear this conclusion in mind.
Yet unless it be thoroughly engrained in the mind, the whole economy of nature, with every fact on distribution, rarity, abundance, extinction, and variation, will be dimly seen or quite misunderstood.
We behold the face of nature bright with gladness, we often see superabundance of food; we do not see or we forget, that the birds which are idly singing round us mostly live on insects or seeds, and are thus constantly destroying life; or we forget how largely these songsters, or their eggs, or their nestlings, are destroyed by birds and beasts of prey; we do not always bear in mind, that, though food may be now superabundant, it is not so at all seasons of each recurring year.
|03 - Struggle for Existence||03-03 - Geometrical Ratio of Increase||10||
A struggle for existence inevitably follows from the high rate at which all organic beings tend to increase.
Every being, which during its natural lifetime produces several eggs or seeds, must suffer destruction during some period of its life, and during some season or occasional year, otherwise, on the principle of geometrical increase, its numbers would quickly become so inordinately great that no country could support the product.
Hence, as more individuals are produced than can possibly survive, there must in every case be a struggle for existence, either one individual with another of the same species, or with the individuals of distinct species, or with the physical conditions of life.
It is the doctrine of Malthus applied with manifold force to the whole animal and vegetable kingdoms; for in this case there can be no artificial increase of food, and no prudential restraint from marriage.
Although some species may be now increasing, more or less rapidly, in numbers, all cannot do so, for the world would not hold them.