| OCR Text |
Show 398 RECAPITULATION. [CBA.P. XIV. CI-IAPTER X IV. RECAPITULATION AND CONCLUSION. Recapitulation of the difficulties on the theory of Natural Selection-Recapitulation of the general :md special circumstances in its favour- Causes of the general b lief in the immutability of species-How far the theory of natural selection may b: extended- Effects of its adoption on the study of Natural History- Concluding remarks. As this whole volume is one long argument, it n1ay be convenient to the reader to have the leading facts and inferences briefly recapitulated. That many and ~rave objec~ions m~:y b~ advanced aO'ainst the theory of descent with mod1ficahon through n~tural selection I do not deny._ I have endeavoured to give to them their full force. Nothing at :first can appear more difficult to believe than that the more complex organs and instincts should have been J?erfected, not by means superior to, though ~nalogous With .human :e~son, but by the accumulation of Innumerable shght vanahon~, each good for the individual possessor. Nevertheless, th1s difficulty though appearing to our imagination insuperably great: cannot be considered real, ifw.e ad~it the following propositions, namely-,-. that gr~dahons 111 the pe;fe~tion of any or gal? or Instinct, whiCh. we may considei, either do now· exist or could have existed, each good of its kind -that all organs and instincts are, in ever so s-light a degree, variable,-and, lastly, that. there is a struggle for exi~tel?-ce leading to the pr~seryatlon of each profitable deviation of structure or Instinct. The truth of these propositions cannot, I think, be disputed. It is, no doubt, extremely difficult even to conjecture by what gradations many structures hay~ been perfected, more especially an1ongst broken and fa1hng groups of or- CBA.P. XIV.] RECAPITULATION. 399 ganic beings ; but ~e see so n1any strange gradations in nature, ,~s IS proclaimed by the canon, " Natura non facit saltum, that we ought to be extremely cautious I·n s · tl t . . aying 1a any_ organ ~r Instinct, or any whole being, could not have ar~Iv~d at Its present_state by n1any graduated steps. There a1e, It Inust be admitted, cases of special difficulty on ~he theory of _natural selection ; and one of tho most curious of these Is the existence of two or three defined castes ~f workers or sterile females in the same community of ants, but I have attempted to show how this difficulty can be mastered. . With respect to the alm~st universal sterility of spemes when .first crossed, ·whiCh forms so remarkable a contrast With the almost universal fertility of varieties '!hen crossed, I m_ust refer the reader to the recapitulatw~ of the facts given at. the end of the eighth chapter, :vhwh seem to me ?onclus1 vely to show that this sterility Is no more a special endowment than is the incapacity of two trees to be grafted together; but that it is incidental on constitutional differences in the reproductive systems of the intercrossed species. We see the truth of this conclusion in the vast difference in the result when the same two species are crossed reciprocally· that is when one species is :first used as the father and then a~ the n1other. The fertility of varieties when intercrossed and of their mongrel offspring cannot be considered as universal· nor is their v_ery- gener~l fertility s_urprising when we re~ember that 1t Is not hkely that either their constitutions or thei~ reproductive systems should have been profoundly modified. Moreover, most of the varieties which have been. exl?erirnentised on have been produced under dom~ st.watlon ; . a.nd as domestication apparently tends to elnnmate sterihty, ·we ought not to expect it also to produce sterility. The sterility of hybrids is a very different case from that of first crosse~, for th.eir reproductive organs are more or less functionally Impotent; whereas in :first crosses the organs on both sides are in a perfect condition. As we continually see that organisms of all kinds |
