The six descendants from (I) will form two sub-genera or even genera. But as the original species (I) differed largely from (A), standing nearly at the extreme points of the original genus, the six descendants from (I) will, owing to inheritance, differ considerably from the eight descendants from (A); the two groups, moreover, are supposed to have gone on diverging in different directions. The intermediate species, also (and this is a very important consideration), which connected the original species (A) and (I), have all become, excepting (F), extinct, and have left no descendants. Hence the six new species descended from (I), and the eight descended from (A), will have to be ranked as very distinct genera, or even as distinct sub-families.

Thus it is, as I believe, that two or more genera are produced by descent, with modification, from two or more species of the same genus. And the two or more parentspecies are supposed to have descended from 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 F, 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 parentspecies (A) and (I), now supposed to be distinct 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 (F) 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.

14

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 a" to p", those marked b" and, and those marked o1 to m1, 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 for further variation and improvement. Within the same large group, the later and more highly perfected subgroups, 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 subgroups. 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.

A distinguished naturalist has objected that the continued action of natural selection and divergence will tend to make an indefinite number of specific forms. 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 the most important, and as the number of species in any country goes on increasing, the organic conditions of life will become more and more complex. Consequently, there seems at first to be no limit to the amount of profitable diversification of structure, and therefore no limit to the number of species which might be produced. We do not know that even the most prolific area is fully stocked with specific forms; at the Cape of Good Hope, which supports such an astonishing number of species, many European plants have become naturalised. But geology shows us, at least within the whole immense Tertiary period, that the number of species of shells, and probably of mammals, has not increased. What, then, checks an indefinite increase in the number of species? Firstly, the amount of life (I do not mean the number of specific forms) supported on any area must have a limit, depending so largely as it does on physical conditions: therefore where very many species are supported, each, or nearly each, will be few in individuals; and any species with scanty numbers would be liable to extermination from accidental fluctuations in the nature of seasons and in the number of its enemies. The process of extermination would in such cases be rapid, whereas the process of the production of new species would always be slow. Imagine the extreme case of as many species as individuals in England, and the first severe winter or very dry summer would exterminate thousands on thousands of species; and individuals of other species would take their places. Secondly, I suspect that when any species becomes very rare, close interbreeding will tend to exterminate it; at least authors have thought that this comes into play in accounting for the deterioration of aurochs in Lithuania, of red deer in Scotland, and of bears in Norway, &c. Thirdly, as far as animals are concerned, some species are closely adapted to prey on some one other being; but if this other being had been rare, it would not have been any advantage to the animal to have been produced in close relation to its prey; therefore it would not have

been produced by natural selection. Fourthly, when any species becomes few in number, the process of modification will be slower, for the chance of favourable variations arising will be lessened; therefore if we suppose an area to be inhabited by very many species, each, or nearly each, species will be poor in individuals, and consequently the process of modification and of giving birth to new forms will be retarded. Fifthly, and this I am inclined to think is the most important element, a dominant species which has already beaten many competitors in its own home, will tend to spread and supplant many others. Alph. de Candolle has shown that those species which spread widely, tend generally to spread very widely; and consequently they will tend to exterminate several species in several areas, and thus check the inordinate increase of specific forms throughout the world. Hooker has recently shown that in the S. E. corner of Australia, where apparently there are many invaders from different quarters of the world, the endemic Australian species have apparently been greatly reduced in number. How much weight to attribute to these several causes, I do not pretend to assign; but conjointly I think they must limit in each country the tendency to an indefinite augmentation of specific forms.

Natural Selection acts, as we have seen, exclusively by the preservation and accumulation of variations, which are beneficial under the organic and inorganic conditions of life to which each creature is at each successive period exposed. The ultimate result will be that each creature will tend to become more and more improved in relation to its conditions of life. This improvement will, I think, inevitably lead to the gradual advancement of the organisation of the greater number of living beings throughout the world. But here we enter on a very intricate subject, for naturalists have not defined to each other's satisfaction what is meant by an advance in organisation. Amongst the Vertebrata, the degree of intellect and an approach in structure to man clearly come into play. It might be thought that the amount of change which the various parts and organs undergo in their development