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m some one species of an earlier genus。 In our diagram； this is indicated by the broken lines； beneath the capital letters； converging in sub…branches downwards towards a single point； this point representing a single species； the supposed single parent of our several new sub…genera and genera。
It is worth while to reflect for a moment on the character of the new species F14； which is supposed not to have diverged much in character； but to have retained the form of （F）； either unaltered or altered only in a slight degree。 In this case； its affinities to the other fourteen new species will be of a curious and circuitous nature。 Having descended from a form which stood between the two parent…species （A） and （I）； now supposed to be extinct and unknown； it will be in some degree intermediate in character between the two groups descended from these species。 But as these two groups have gone on diverging in character from the type of their parents； the new species （F14） will not be directly intermediate between them； but rather between types of the two groups； and every naturalist will be able to bring some such case before his mind。
In the diagram； each horizontal line has hitherto been supposed to represent a thousand generations； but each may represent a million or hundred million generations； and likewise a section of the successive strata of the earth's crust including extinct remains。 We shall； when we come to our chapter on Geology； have to refer again to this subject； and I think we shall then see that the diagram throws light on the affinities of extinct beings； which； though generally belonging to the same orders； or families； or genera； with those now living； yet are often； in some degree； intermediate in character between existing groups； and we can understand this fact； for the extinct species lived at very ancient epochs when the branching lines of descent had diverged less。
I see no reason to limit the process of modification； as now explained； to the formation of genera alone。 If； in our diagram； we suppose the amount of change represented by each successive group of diverging dotted lines to be very great； the forms marked a214 to p14； those marked b14 and f14； and those marked o14 to m14； will form three very distinct genera。 We shall also have two very distinct genera descended from （I） and as these latter two genera； both from continued divergence of character and from inheritance from a different parent； will differ widely from the three genera descended from （A）； the two little groups of genera will form two distinct families； or even orders； according to the amount of divergent modification supposed to be represented in the diagram。 And the two new families； or orders； will have descended from two species of the original genus； and these two species are supposed to have descended from one species of a still more ancient and unknown genus。
We have seen that in each country it is the species of 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 tend to 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 as yet have suffered least extinction； will for a long period continue to increase。 But which groups will ultimately prevail； no man can predict； for we well 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 on this view of extremely few of the more ancient species having transmitted descendants； and on the view of all the descendants of the same species making a class； we can understand how it is that there exist but very few classes in each main division of the animal and vegetable kingdoms。 Although extremely few of the most ancient species may now have living and modified descendants； yet at the most remote geological period； the earth may have been as well peopled with many species of many genera； families； orders； and classes； as at the present day。
Summary of Chapter
If during the long course of ages and under varying conditions of life； organic beings vary at all in the several parts of their organisation； and I think this cannot be disputed； if there be； owing to the high geometrical powers of increase of each species； at some age； season； or year； a severe struggle for life； and this certainly cannot be disputed； then； considering the infinite complexity of the relations of all organic beings to each other and to their conditions of existence； causing an infinite diversity in structure； constitution； and habits； to be advantageous to them； I think it would be a most extraordinary fact if no variation ever had occurred useful to each being's own welfare； in the same way as so many variations have occurred useful to man。 But if variations useful to any organic being do occur； assuredly individuals thus characterised will have the best chance of being preserved in the struggle for life； and from the strong principle of inheritance they will tend to produce offspring similarly characterised。 This principle of preservation； I have called； for the sake of brevity； Natural Selection。 Natural selection； on the principle of qualities being inherited at corresponding ages； can modify the egg； seed； or young； as easily as the adult。 Amongst many animals； sexual selection will give its aid to ordinary selection； by assuring to the most vigorous and best adapted males the greatest number of offspring。 Sexual selection will also give characters useful to the males alone； in their struggles with other males。
Whether natural selection has really thus acted in nature； in modifying and adapting the various forms of life to their several conditions and stations； must be judged of by the general tenour and balance of evidence given in the following chapters。 But we already see how it entails extinction；