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Show 8 INTRODUCTION. of the surrounding physical conditions and by correlation of growth. On the principles here briefly sketched out, there is no innate or neces ary tendency in each being to its own ad vancement in the scale of organization. vVe are almost compelled to look at the specialization or differentiation of parts or organs for different fun tions as the best or even sole stanrlard of ad van cement ; for by such division of labour each function of body and mind is better performed. And, as natmal selection acts exclusively through the preservation of profitable modifications of structure, and as the conditions of life in each area generally become more and more complex, from the increasing nnmber of different furms which inhabit it and from most of these forms acquiring a more and more perfect strudure, we may confidently bel!eve, that, on the whole, organization advances. Nevertheless a very simple form fitted for very simple conditions of life might remain for indefinite ages unalter d or unimproved ; for what wonld it profit an infusorial animalcul e, for instance, or an inte tinal worm, to become highly organized? Members of a high gronp might even become, and thi apparently has occurred, fitted for simpler conditions of life; and in this case natural selection would tend to simplify or degrade tile organization, for complicated mechanism for simple actions would. be usele s or even di advantageous. In a second work, after treating of the Variation of organisms in a state of nature, of the Struggle for Exi tence and the prinC'iple of Natural Selection, I hall discuss the difficulties which are opposed to the th eory. The e difficulties may be cla, ed under the following heads :-the apparent impossibility in some cases of a very simple organ graduating by small step into a highly perfect organ; the manellous fact of Insti11ct ; the whole question of H) bridity; aud, lastly, the ab ence, at the pr ~ ent time and in our geological formation , of innumerable link connecting all allied species. Althongh some of these diffi cultie are of great weight, we hall see that many of them are explicable on the th eory of natural election, and are otherwi e in explicable. In scientific inve ti~ation it i:s permitted to inYeut any hypothe is, and if it explains various large and ind pendent cla scs of facts it rises to the rank of a well-grounded theory. The NATURAL SELECTION. 9 undulations of the ether and even its existence are hypoth etical, yet every one now admits the undulatory theory of light. The principle of natural selection may be looked at as a mere hypothe is, but rendered in some degree probable by what we positively know of the variability of organic beings in a state of nature,-by what we positively know of the struggle for exi tence, a11d the con seq nent almost inevitable preservation of favonrable variations,-and from the analogical formation of domestic races. Now this hypothesis may be test cl,--and this seems to me the only fair and legitimate ma1mer of considering the whole qnestion,-by trying whether it explains several large and independent classes of facts; such as the geological succession of organic beings, their distribution in past and present times, and their mutual affinities and homologies. If the principle of natural selection does explain these and other large bodies of fad s, it onght to be received. On the ordinary view of each species having been independently created, we gain no scientific explanation of any one of these facts. 'vVe ean only say that it has so pleased the Creator to command that the past and present inhabitants of the world shonld appear in a certain order and. in certain areas; that He has impressed on them the most extrnordinary resemblances, and has clas ·ed them in groups subordinate to gronps. .But by such statements we gain no new knowledge ; we do not connect together facts andla,<'s; we explain nothing. In a third work I shall try the principle of natural selection by seeing bow far it will give a fair explanation of the several cia ses of facts just alluded to. It was the consideration of these facts which tlrst led me to take up the present su~ject. ·when I visited, during the voyage of H.M.S. Beagle, the Galapagos Archipelago, situated in 1he Pacific Ocean about 500 miles from the shore of South America, I found myself surrounded by peculiar species of birds, reptiles, and plants, existing nowhere else in the world. Yet they nearly all bore an American stamp. In the song of the mocking-thrush, in the harJ1 cry of the carrion-hawk, in the great candlestick-like opuntias, I clearly perceived the neighbourhood of America, though the islands were separated by so many miles of ocean from the mainland, and differed much from it in their geolQgical |