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04 - Natural Selection 04-10 - Extinction caused by Natural Selection 20 We have seen that the species which are most numerous in individuals have the best chance of producing favourable variations within any given period.

We have evidence of this, in the facts stated in the second chapter showing that it is the common and diffused or dominant species which offer the greatest number of recorded varieties.

Hence, rare species will be less quickly modified or improved within any given period; they will consequently be beaten in the race for life by the modified and improved descendants of the commoner species.
04 - Natural Selection 04-14 - Summary of Chapter 30 We have seen that it is the common, the widely-diffused and widely-ranging species, belonging to the larger genera within each class, which vary most; and these tend to transmit to their modified offspring that superiority which now makes them dominant in their own countries.

Natural selection, as has just been remarked, leads to divergence of character and to much extinction of the less improved and intermediate forms of life.

On these principles, the nature of the affinities, and the generally well-defined distinctions between the innumerable organic beings in each class throughout the world, may be explained.

It is a truly wonderful fact- the wonder of which we are apt to overlook from familiarity- that all animals and all plants throughout all time and space should be related to each other in groups, subordinate to groups, in the manner which we everywhere behold- namely, varieties of the same species most closely related, species of the same genus less closely and unequally related, forming sections and sub-genera, species of distinct genera much less closely related, and genera related in different degrees, forming sub-families, families, orders, sub-classes and classes.

The several subordinate groups in any class cannot be ranked in a single file, but seem clustered round points, and these round other points, and so on in almost endless cycles.

If species had been independently created, no explanation would have been possible of this kind of classification; but it is explained through inheritance and the complex action of natural selection, entailing extinction and divergence of character, as we have seen illustrated in the diagram.
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04 - Natural Selection 04-11 - Divergence of Character 260 We have seen that in each country it is the species belonging to the larger genera which oftenest present varieties or incipient species.

This, indeed, might have been expected; for, as natural selection acts through one form having some advantage over other forms in the struggle for existence, it will chiefly act on those which already have some advantage; and the largeness of any group shows that its species have inherited from a common ancestor some advantage in common.

Hence, the struggle for the production of new and modified descendants will mainly lie between the larger groups which are all trying to increase in number.

One large group will slowly conquer another large group, reduce its numbers, and thus lessen its chance of further variation and improvement.

Within the same large group, the later and more highly perfected sub-groups, from branching out and seizing on many new places in the polity of Nature, will constantly tend to supplant and destroy the earlier and less improved sub-groups. Small and broken groups and sub-groups will finally disappear.

Looking to the future, we can predict that the groups of organic beings which are now large and triumphant, and which are least broken up, that is, which have as yet suffered least extinction, will, for a long period, continue to increase. But which groups will ultimately prevail, no man can predict; for we know that many groups formerly most extensively developed, have now become extinct.

Looking still more remotely to the future, we may predict that, owing to the continued and steady increase of the larger groups, a multitude of smaller groups will become utterly extinct, and leave no modified descendants; and consequently that, of the species living at any one period, extremely few will transmit descendants to a remote futurity.

I shall have to return to this subject in the chapter on Classification, but I may add that as, according to this view, extremely few of the more ancient species have transmitted descendants to the present day, and, as all the descendants of the same species form a class, we can understand how it is that there exist so few classes in each main division of the animal and vegetable kingdoms.

Although few of the most ancient species have left modified descendants' yet, at remote geological periods, the earth may have been almost as well peopled with species of many genera, families, orders, and classes, as at the present time.

Charles Darwin
Charles Darwin
04 - Natural Selection 04-01 - Natural Selection 50 We have good reason to believe, as shown in the first chapter, that changes in the conditions of life give a tendency to increased variability; and in the foregoing cases the conditions have changed, and this would manifestly be favourable to natural selection, by affording a better chance of the occurrence of profitable variations.

Unless such occur, natural selection can do nothing.

Under the term of "variations," it must never be forgotten that mere individual differences are included.