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forms or geographical races, crossing cannot come into play. It should also be borne in mind that the offspring from a cross between a modified and unmodified species tends partially to inherit the characters of both parents, and natural selection assuredly will preserve even slight approaches to any change of structure which is beneficial. Moreover such crossed offspring, from partaking of the same constitution with the modified parent, and from being still exposed to the same conditions, will be far more liable than other individuals of the same species again to vary or be modified in a similar manner.

It has been argued that, as none of the animals and plants of Egypt, of which we know anything, have changed during the last 3000 years, so probably none have been modified in any other part of the world. The many animals which have remained unchanged since the commencement of the glacial period would have been an incomparably stronger case, for these have been exposed to great changes of climate and have migrated over great distances; whereas, in Egypt, during the last 3000 years, the conditions of life, as far as we know, have remained absolutely uniform. The fact of little or no modification having been effected since the glacial period would be of some avail against those who believe in the existence of an innate and necessary law of development, but is powerless against the doctrine of natural selection, which implies only that variations occasionally occur in single species, and that these when favourable are preserved; but this will occur only at long intervals of time after changes in the conditions of each country. As Mr. Fawcett has well asked, what would be thought of a man who argued that, because he could show that Mont Blanc and the other Alpine peaks had exactly the same height 3000 years ago as at present, consequently that these mountains

had never been slowly upraised, and that the height of other mountains in other parts of the world had not recently been increased by slow degrees?

It has been objected, if natural selection be so powerful, why has not this or that organ been recently modified and improved? Why has not the proboscis of the hive-bee been lengthened so as to reach the nectar of the red-clover? Why has not the ostrich acquired the power of flight? But granting that these parts and organs have varied in the right directiongranting that there has been time sufficient for the slow work of natural selection, checked as it will be by intercrossing and the tendency to reversion, who will pretend that he knows the natural history of any one organic being sufficiently well to say that any particular change would on the whole be to its advantage? Can we feel sure that a long proboscis would not be a disadvantage to the hive-bee in sucking the innumerable small flowers which it frequents? Can we feel sure that a long proboscis would not, by correlation of growth, almost necessarily give increased size to other parts of the mouth, perhaps interfering with the delicate cell-constructing work? In the case of the ostrich, a moment's reflection. will show that an enormous supply of food would be necessary in this bird of the desert, to supply force to move its huge body through the air. But such ill-considered objections are hardly worth notice.

The celebrated paleontologist, Professor Bronn, in his German translation of this work, has advanced various good objections to my views, and other remarks in its favour. Of the objections, some seem to me unimportant, some few are owing to misapprehension, and some are incidentally noticed in various parts of this volume. On the erroneous supposition that all the species of a region are believed by me to be changing

at the same time, he justly asks how it is that all the forms of life do not present a fluctuating and inextricably confused body? but it is sufficient for us if some few forms at any one time are variable, and few will dispute that this is the case. He asks, how, on the principle of natural selection, can a variety live in abundance side by side with the parent-species; for a variety during its formation is supposed to supplant the intermediate forms between itself and the parentspecies, and yet it has not supplanted even the parentspecies, for both now live together? If the variety and parent-species have become fitted for slightly different habits of life, they might live together; though, in the case of animals which freely cross and move about, varieties seem to be almost always confined to distinct localities. But is it the case that varieties of plants and of the lower animals are often found in abundance side by side with the parent-forms? Laying aside polymorphic species, in which innumerable variations seem neither advantageous nor disadvantageous to the species, and have not been fixed; laying aside also temporary variations, such as albinism, &c., my impression is that varieties and the supposed parent-species are generally found inhabiting distinct stations, high land or low land, dry or moist districts, or distinct regions.

Again, Professor Bronn truly remarks, that distinct species do not differ from each other in single characters alone, but in many; and he asks, how it comes that natural selection should always have simultaneously affected many parts of the organisation? Probably the whole amount of difference has not been simultaneously effected; and the unknown laws of correlation will certainly account for, but not strictly explain, much simultaneous modification. Anyhow, we see in our domestic varieties the very same fact: though our do

mestic races may differ much in some one organ from the other races of the same species, yet the remaining parts of the organisation will always be found in some degree different. Professor Bronn likewise asks with striking effect how, for instance in the mouse or hare genus, can natural selection account for the several species (descended, I may remark, from a parent of unknown character) having longer or shorter tails, longer or shorter ears, and fur of different colours; how can it account for one species of plant having pointed and another species blunt leaves? I can give no definite answer to such questions; but I might ask in return, were these differences, on the doctrine of independent creation, formed for no purpose? If of use, or if due to correlation of growth, they could assuredly be formed through the natural preservation of such useful or correlated variations. I believe in the doctrine of descent with modification, notwithstanding that this or that particular change of structure cannot be accounted for, because this doctrine groups together and explains, as we shall see in the later chapters, many general phenomena of nature.

A distinguished botanist, Mr. H. C. Watson, believes that I have overrated the importance of the principle of divergence of character (in which, however, he apparently believes), and that convergence of character, as it may be called, has likewise played a part. This is an intricate subject which need not be here discussed. I will only remark that if two species of two closely allied genera produced a number of new and divergent species, I can believe that these new forms might sometimes approach each other so closely that they would for convenience' sake be classed in the same new genus, and thus two genera would converge into one; but from the strength of the principle of inheritance, it

seems hardly credible that the two groups of new species would not at least form two sections of the supposed new single genus.

Mr. Watson has also objected that the continued action of natural selection with divergence of character 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 more important; and as the number of species in any country goes on increasing, the organic conditions of life become more and more complex. Consequently there seems at first sight 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 and in Australia, which support such an astonishing number of species, many European plants have become naturalised. But geology shows us, that from an early part of the long tertiary period the number of species of shells, and that from the middle part of this same period the number of mammals, has not greatly or at all increased. What then checks an indefinite increase in the number of species? 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, if an area be inhabited by very many species, each or nearly each species will be represented by few individuals; and such species will be liable to extermination from accidental fluctuations in the nature of the seasons or in the number of their enemies. The process of extermination in these cases

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