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links； is a very obvious difficulty。 I assigned reasons why such links do not commonly occur at the present day； under the circumstances apparently most favourable for their presence； namely on an extensive and continuous area with graduated physical conditions。 I endeavoured to show； that the life of each species depends in a more important manner on the presence of other already defined organic forms； than on climate； and； therefore； that the really governing conditions of life do not graduate away quite insensibly like heat or moisture。 I endeavoured； also； to show that intermediate varieties； from existing in lesser numbers than the forms which they connect； will generally be beaten out and exterminated during the course of further modification and improvement。 The main cause； however； of innumerable intermediate links not now occurring everywhere throughout nature depends on the very process of natural selection； through which new varieties continually take the places of and exterminate their parent…forms。 But just in proportion as this process of extermination has acted on an enormous scale； so must the number of intermediate varieties； which have formerly existed on the earth； be truly enormous。 Why then is not every geological formation and every stratum full of such intermediate links？ Geology assuredly does not reveal any such finely graduated organic chain； and this； perhaps； is the most obvious and gravest objection which can be urged against my theory。 The explanation lies； as I believe； in the extreme imperfection of the geological record。
In the first place it should always be borne in mind what sort of intermediate forms must； on my theory； have formerly existed。 I have found it difficult； when looking at any two species； to avoid picturing to myself； forms directly intermediate between them。 But this is a wholly false view； we should always look for forms intermediate between each species and a common but unknown progenitor； and the progenitor will generally have differed in some respects from all its modified descendants。 To give a simple illustration： the fantail and pouter pigeons have both descended from the rock…pigeon； if we possessed all the intermediate varieties which have ever existed； we should have an extremely close series between both and the rock…pigeon； but we should have no varieties directly intermediate between the fantail and pouter； none； for instance； combining a tail somewhat expanded with a crop somewhat enlarged； the characteristic features of these two breeds。 These two breeds； moreover； have become so much modified； that if we had no historical or indirect evidence regarding their origin； it would not have been possible to have determined from a mere comparison of their structure with that of the rock…pigeon； whether they had descended from this species or from some other allied species； such as C。 oenas。
So with natural species； if we look to forms very distinct； for instance to the horse and tapir； we have no reason to suppose that links ever existed directly intermediate between them； but between each and an unknown common parent。 The common parent will have had in its whole organisation much general resemblance to the tapir and to the horse； but in some points of structure may have differed considerably from both； even perhaps more than they differ from each other。 Hence in all such cases； we should be unable to recognise the parent…form of any two or more species； even if we closely compared the structure of the parent with that of its modified descendants； unless at the same time we had a nearly perfect chain of the intermediate links。
It is just possible by my theory； that one of two living forms might have descended from the other； for instance； a horse from a tapir； and in this case direct intermediate links will have existed between them。 But such a case would imply that one form had remained for a very long period unaltered； whilst its descendants had undergone a vast amount of change； and the principle of competition between organism and organism； between child and parent； will render this a very rare event； for in all cases the new and improved forms of life will tend to supplant the old and unimproved。
By the theory of natural selection all living species have been connected with the parent…species of each genus； by differences not greater than we see between the varieties of the same species at the present day； and these parent…species； now generally extinct； have in their turn been similarly connected with more ancient species； and so on backwards； always converging to the common ancestor of each great class。 So that the number of intermediate and transitional links； between all living and extinct species； must have been inconceivably great。 But assuredly； if this theory be true； such have lived upon this earth。
On the lapse of Time。 Independently of our not finding fossil remains of such infinitely numerous connecting links； it may be objected； that time will not have sufficed for so great an amount of organic change； all changes having been effected very slowly through natural selection。 It is hardly possible for me even to recall to the reader； who may not be a practical geologist； the facts leading the mind feebly to comprehend the lapse of time。 He who can read Sir Charles Lyell's grand work on the Principles of Geology； which the future historian will recognise as having produced a revolution in natural science； yet does not admit how incomprehensibly vast have been the past periods of time； may at once close this volume。 Not that it suffices to study the Principles of Geology； or to read special treatises by different observers on separate formations； and to mark how each author attempts to give an inadequate idea of the duration of each formation or even each stratum。 A man must for years examine for himself great piles of superimposed strata； and watch the sea at work grinding down old rocks and making fresh sediment； before he can hope to comprehend anything of the lapse of time； the monuments of which we see around us。
It is good to wander along lines of sea…coast； when formed of moderately hard rocks； and mark the process of degradation。 The tides in most cases reach the cliffs only for a short time twice a day； and the waves eat into them only when they are charged with sand or pebbles； for there is reason to believe that pure water can effect little or nothing in wearing away rock。 At last the base of the cliff is undermined； huge fragments fall down； and these remaining fixed； have to be worn away； atom by atom； until reduced in size they can be rolled about by the waves； and then are more quickly ground into pebbles； sand； or mud。 But how often do we see along the bases of retreating cliffs rounded boulders； all thickly clothed by marine productions； showing how little they are abraded and how seldom they are rolled about！ Moreover； if we follow for a few miles any line of rocky cliff； which is undergoing degradation； we find that it is only here and there； along a short length or round a promontory； that the cliffs are at the present time suffering。 The appearance of the surface and the vegetation show that elsewhere years have elapsed since