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Show 218 ISLAND LIFE. (PAitT I. denudation is effected by the waves of the ocean eating away coast lines. This was once thought to be of more importance than sub-aerial denudation, but it is now believed to be comparatively slow in its action. 1 Whatever it may be,. howev.er,. it adds to the rate of formation of new strata, and 1ts om1sswn from the calculation is again on the side of making the lapse of time greater rather than less than the true amount. Even if a considerable modification should be needed in some of the assumptions it has been necessary to make, the result must still show that, so· far as the time required for the formation of the known stratified rocks, the hundred million years allowed by physicists is not only ample, but will permit of even more than an equal period anterior to the lowest Cambrian rocks, as demanded by Mr. Darwin-a demand supported and enforced by the arguments, taken from independent standpoints, of Professor Huxley and Professor Ramsay. Organic Modification dependent on Change of Conditions.Having thus shown that the physical changes of the earth's surface may have gone on much more rapidly and occupied much less time than has generally been supposed, we have now to inquire whether there are any considerations which lead to the conclusion that organic changes may have gone on with corresponding rapidity. There is no part of the theory of natural selection which is more clear and satisfactory than that which connects cl~anges of ascertained fact, that the area of deposition is many times smaller than the area of denudation. 1 Dr. Croll and Professor Geilde have shown that marine denudation is very small in amount as compared with sub-aerial, since it acts only locally on the edge of the land, whereas the latter acts over every foot of the .<;urface. Mr. W. 'f. Blanford argues that the difference is still greater in tropical than in temperate latitudes, and arrives at the conclusion that" If over British India the effects of marine to those of fresh-water denudation in removing the rocks of the country be estimated at 1 to 100, I believe that the result of marine action· will be greatly overstated'' ( Geology and Zoology of Abyssinia, p. 158, note). Now, as our estimate of the rate of sub-aerial denudation cannot pretend to any precise accuracy, we are justified in neglecting marine denudation altogether, especially as we have no method of estimating it for the whole earth with any approach to correctness. rnAr. x.] THE llATE OF OllGANlC CIIA~GE. 219 specific forms with changes of external conditions or environment. If the external world remains for a moderate period unchanged, the organic world soon reaches a state of equilibrium throug? the struggle for existence; each species occupies its place m nature, and there is then no inherent tendency to change. But almost any change whatever in the external world disturbs this equilibrium, and may set in motion a whole series of organic revolutions before it is restored. A chancre of climate in any direction will be sure to injure some and be:efit ?ther sp~cies. The one will consequently diminish, the other mcrease m number ; and the former may even become extinct. But the extinction of a species will certainly affect other species which it either preyed upon, or competed with, or served for food; while the increase of any one animal may soon lead to the extinction of some other to which it was inimical. These changes will in their turn bring other changes ; and before an equilibrium is again established, the proportions, ranges, and numbers, of the species inhabiting the country may be materially altered. The complex manner in which animals are related to each other is well exhibited by the importance of insects which in many parts of the world limit the numbers or deter~ mine the very existence ~f some of the higher animals. Mr. Darwin says :-r-" Perhaps Paraguay offers the most curious instance of this ; for here neither cattle, nor horses, nor dogs have ever run wild, though they swarm southward and northward in a wild state; and Azara and Rengger have shown that this is caused by the greater number in Paraguay of a certain fly, which lays its eggs in the navels of these animals when first born. The increase of these flies, numerous as they are, must be habitually checked by some means, probably by other parasitic insects. Hence, if certain insectivorous birds were to decrease in Paraguay, the parasitic insects would probably i~crease; and this would lessen the number of navel-frequenting :fhes-then cattle and horses would run wild ; and this would certainly alter (as indeed I have observed in parts of South America) the vegetation: this again would largely affect the insects, and this, as we have seen in Staffordshire, the insectivorous birds, and so onwards in ever increasing circles of complexity." |