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04 - Natural Selection 04-09 - Circumstances favourable for the production of new forms through Natural Selection 120 That natural selection generally acts with extreme slowness I fully admit. It can act only when there are places in the natural polity of a district which can be better occupied by the modification of some of its existing inhabitants.

The occurrence of such places will often depend on physical changes, which generally take place very slowly, and on the immigration of better adapted forms being prevented.

As some few of the old inhabitants become modified, the mutual relations of others will often be disturbed; and this will create new places, ready to be filled up by better adapted forms, but all this will take place very slowly.

Although the individuals of the same species differ in some slight degree from each other, it would often be long before differences of the right nature in various parts of the organisation might occur.

The result would often be greatly retarded by free intercrossing.

Many will exclaim that these several causes are amply sufficient to neutralise the power of natural selection.

I do not believe so.

But I do believe that natural selection will generally act very slowly, only at long intervals of time, and only on a few of the inhabitants of the same region.

I further believe that these slow, intermittent results accord well with what geology tells us of the rate and manner at
which the inhabitants of the world have changed.

Grand Canyon
Grand Canyon

Slow though the process of selection may be, if feeble man can do much by artificial selection, I can see no limit to the amount of change, to the beauty and complexity of the coadaptations between all organic beings, one with another and with their physical conditions of life, which may have been effected in the long course of time through nature's power of selection, that is by the survival of the fittest.
04 - Natural Selection 04-11 - Divergence of Character 100 The accompanying diagram will aid us in understanding this rather perplexing subject.

Full Size

Let A to L represent the species of a genus large in its own country; these species are supposed to resemble each other in unequal degrees, as is so generally the case in nature, and as is represented in the diagram by the letters standing at unequal distances.

I have said a large genus, because as we saw in the second chapter, on an average more species vary in large genera than in small genera; and the varying species of the large genera present a greater number of varieties.

We have, also, seen that the species, which are the commonest and the most widely diffused, vary more than do the rare and restricted species. Let (A) be a common, widely-diffused, and varying species, belonging to a genus large in its own country.

The branching and diverging lines of unequal lengths proceeding from (A), may represent its varying offspring.

The variations are supposed to be extremely slight, but of the most diversified nature; they are not supposed all to appearsimultaneously, but often after long intervals of time, nor are
they an supposed to endure for equal periods.

Only those variations which are in some way profitable will be preserved or naturally selected.

And here the importance of the principle of benefit derived from divergence of character comes in; for this will generally lead to the most different or divergent variations (represented by the outer lines) being preserved and accumulated by natural selection.

When a line reaches one of the horizontal lines, and is there marked by a small numbered letter, a sufficient amount of variation is supposed to have been accumulated to form it into a fairly well-marked variety, such as would be thought worthy of record in a systematic work.
04 - Natural Selection 04-11 - Divergence of Character 80 The advantage of diversification of structure in the inhabitants of the same region is, in fact, the same as that of the physiological division of labour in the organs of the same individual body- a subject so well elucidated by Milne Edwards.

No physiologist doubts that a stomach adapted to digest vegetable matter alone, or flesh alone, draws most nutriment from these substances.

So in the general economy of any land, the more widely and perfectly the animals and plants are diversified for different habits of life, so will a greater number of individuals be capable of there supporting themselves.

A set of animals, with their organisation but little diversified, could hardly compete with a set more perfectly diversified in structure.

It may be doubted, for instance, whether the Australian marsupials, which are divided into groups differing but little from each other, and feebly representing, as Mr. Waterhouse and others have remarked, our carnivorous, ruminant, and rodent mammals, could successfully compete with these well-developed orders.




In the Australian mammals, we see the process of diversification in an early and incomplete stage of development.
04 - Natural Selection 04-14 - Summary of Chapter 40 The affinities of all the beings of the same class have sometimes been represented by a great tree. I believe this simile largely speaks the truth.

The green and budding twigs may represent existing species; and those produced during former years may represent the long succession of extinct species.

At each period of growth all the growing twigs have tried to branch out on all sides, and to overtop and kill the surrounding twigs and branches, in the same manner as species and groups of species have at all times overmastered other species in the great battle for life.

The limbs, divided into great branches, and these into lesser and lesser branches, were themselves once, when the tree was young, budding twigs, and this connection of the former and present buds by ramifying branches may well represent the classification of all extinct and living species in groups subordinate to groups.

Of the many twigs which flourished when the tree was a mere bush, only two or three, now grown into great branches, yet survive and bear the other branches; so with the species which lived during long-past geological periods very few have left living and modified descendants.

From the first growth of the tree, many a limb and branch has decayed and dropped off; and these fallen branches of various sizes may represent those whole orders, families, and genera which have now no living representatives, and which are known to us only in a fossil state.

As we here and there see a thin straggling branch springing from, a fork low down in a tree, and which by some chance has been favoured and is still alive on its summit, so we occasionally see an animal like the Ornithorhynchus or Lepidosiren, which in some small degree connects by its affinities two large branches of life, and which has apparently been saved from fatal competition by having inhabited a protected station. As buds give rise by growth to fresh buds, and these, if vigorous, branch out and overtop on all sides many a feebler branch, so by generation I believe it has been with the great Tree of Life, which fills with its dead and broken branches the crust of the earth, and covers the surface with its everbranching and beautiful ramifications.