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Show CliAP. VI. DIFFICULTIES ON THEORY. 176 1 ill bably apply to both ; and if we in the same ru e w pro . . . . t' dapt a varying spemes to a very 1a rge area, Imagina lOll a . . 1 . h ll h to adapt two varieties to two arge areas, we s a ave di Th and a third variety to a narrow interm.e at~ zo~e. e I. llterme d' t I'ety consequently, Will eXISt ill lesser Ia e var ' . d b f I'nhabiting a narrow and lesser area, an num ers rom . ld d t . lly as far as I can make out, this rule ho s goo prac ICa , h t 'th wi' th van'e ti'es in a state of nature. I ave f me ·W I· stn'k I' ng I·n s t a nces of the rule in the. ca. se . o vanetieR 1· n t erm edi'ate between well-marked var1et1es In t.h e ge.n us Balanus. And it would appear from information g1ven me by Mr. Watson, Dr. Asa Gray, and ~r. Wollaston, that generally when varieties intermediate be~ween two other forms occur, they are much rarer numen cally than the forms which they connect. Now, if we may trust these facts and inferences, and therefore conclude that varieties linking two other varieties together h~ve generally existed in lesser numbers than the forms whi.ch they connect, then, I think, we can understand why Intermediate varieties should not endure for very long periods ;-why as a general rule they should be ext~rminated and disappear, sooner than the forms wluch they originally linked together For any form existing in lesser numbers .would, as already remarked, run a greater chance of being exte~minated than one existing in large numbers ; and In this particular case the intermediate form ':'ould be eminently liable to the inroads of closely all.wd forms existing on both sides of it. But a far more Important consideration, as I believe, is that, during the process of further modification, by which two varieties are ~upposed on my theory to be converte~ and ~erf~cted In:~ two distinct species, the two whwh exist Ill larg numbers from inhabiting larger areas, will have a gr~at advantage over the intermediate variety, which exists CHAP. VI. TRANSITIONAL V .ARIETIES. 177 in smaller numbers in a narrow and intermediate zone. For forms existing in larger numbers will always have a better chance, within any given period, of presenting further favourable variations for natural selection to seize on, than will the rarer forms which exist in lesser numbers. Hence, the more common forms in the race for life, will tend to beat and supplant the less common forms, for these will be more slowly modified and improved. It is the same principle which, as I believe, accoun~s for the common species in each country, as shown m the second chapter, presenting on an average a greater number of well-marked varieties than do the rar~r species. I may illustrate what I mean by supposing three. varieties of sheep to be kept, one adapted to an. extensive mountainous region; a second to a compar~ tively narrow, hilly tract; and a third to wide plains at the base; and that the inhabitants are all trying with equ~l steadiness and skill to improve their stocks by selection; the chances in this case will be st~ongly in favour of the great holders on the moun-tain. s or on the p1 a 1· ns I·m prov·i ng thei.r breeds more qmckly than the small holders on the intermediate narrow,. hilly tr~ct ; and consequently the improved ~nountai~ or plain breed will soon take the place of he. less ~~proved hill breed ; and thus the two breeds WhiCh OflD'I ll · t d · ' • t:> na Y ex1s e In greater numbers will come Int~ .close contact with each other, without the interpo; twn of the supplanted, intermediate hill-variety. no sum up, I believe that species come to be tolerably we -defi.n ed ob ~' ec t s, an d d o not at any one period pr - sent an Inext · bl h f · . 1. k nca e c aos o varying and Intermediate J.'In 8 : :firstly, because new varieties are very slowly lOrmed ' for vari.a tI' on I.s a very slow process and. tn'a turalh se 1e c t I·o n can do nothing until favourable ' varia- Ions c ance to occur, and until a place in the natural I 3 |