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Show CHAPTER VII ON THE INFERTILITY OF CROSSES BETWEEN DISTINCT SPECIES AND THE USUAL t>TJ!:RILITY OF THEIR HYBRID Ol!'FSPRING f tl . '·1 m Extreme susceptibility of the reproductive Statement o 1e proo r - ~ . functions-Reciprocal crosses-lndividu~l diffe.rences m respect . to cross-fertilisation-Dimorphism aud tnmorp~usm. ~u).()ng planls-c f ll e fertility of hybrids and of the mferbhty of mongrels ases o 1 . H 1 • b' r , F t'l _'!'he effects of close inter-breedmg--Mr. ut 1 s o Jee rous-- er 1 e hybrids among animals-Fertility of hybrids amo~1g plants- Cases o~ t ··l't of rnurwrels-Parallelism between crossurg and cha11ge o[ s en 1 y b · · S T t 1 l conditions-Remarks on the facts of hybn~hty- ten 1 Y l ne o c1 r ange d con d1't '1o ns and usually correlated wt.t h. .. o. ther• ,( 'har.a ct1e rs· - Correlation of colour with constitutional pecuhanttCs- 1 he rso at~on of varieties by selective association-The influenco_of 11atnral sclectwn upon sterility and fertility-Physiologieal selecbon-Summary and concluding remarks. ONE of the greatest, or perhaps we may ~ay tho greatest, of all the difficulties in tho way of acccptmg the the?r:Y of natural selection as a complete explanation of the ong~n _of species, has been the remarka?l~ difference between van~.t1cs and species in respect of fert1hty when crossed. Goner ,~lly speaking, it may be said that the varieties of any one spceu~s, however different they may be in external appea.~·a.n~e, arc perfectly fertile when crossed, and their mongrel o~sprn:g. ~' re equally fertile when bred among themselves ; while chstmct species, on the other hand, howeve~ clos~ly they may resemble each other externally, are usually mfcrt1lc whe~ crossed, an<l their hybrid offspring absolutely sterile. This used to he considered a fixed law of nature, constituting the absolute tm;t and criterion of a specie.s as distinct from a vm·ifty /. and so long as it was believed that species were separate creatwns, or CHAP. VII ON TilE INFERTILITY OF CROS ES 153 at _all cv •11ts had ftn origin quite distinct from that of varieties, th1s bw eould h:~vc 110 exceptions, because, if ~ny two species had ~con found to be fertile when crossed and their hybrid offspnng to be :~lso fertile, this fact wonld h;~vc been. held to prove them to he not species but 'va1·ieties. On the other hand if two. varieties had been found to be infertile, or their mongrel o:ffsprmg to he sterile, then it would have been said: These are not varieties hut true species. Thus the old theory led to inevitable reasoning in a circle; and what might be only a rather common fact was elevated into a law which had no exceptions. The clahon:tc n.nd careful examination of the whole subject by Mr. Danvm, ·who has brought together a va. t mass of evidence from the experience of agricultnrists and horticulturists, as well as from scientific experimenters, has demonstrated that there is no such fixed lu.w in nature as was formerly supposed. He shows us that eros. c. between some varieties are infertile or even sterile, while crosses between some species arc quito fertile; and that there arc besides a number of curious phenomena connected with the subject which render it impossible to believe that sterility is anything more than an incidental property of species, due to the extreme delicacy and susceptibility of the reproductive powers, and dependent on physiological causes we have not yet been ahl? to trace .. N cvertheles., the fact remains that most species ~h1ch have h1therto been crossed prodnce sterile hybrids, as m the well-known case of the mule; while almo t all domestic vari_eties, when crossed, produce offspring wl1ich arc perfectly fcrtJ]c among thcms:lvcs. I will 110w endeavour to give such a sketch of the subJect as may enable the reader to see something of the complexity· of the problem, referring him to Mr. Darwin's work!:> for fuller details. Extreme Susceptibility nf the Reproductive Functions. One of tho mo:-;t interesting facts, a.s showing how susceptible to changed conditions or to slight constitutional changes are the reproductive powers of animals, is the very general difficulty of getting those which are kept in confinemont to breed; and this is frequently the only bar to domesticating wild species. Thus, elephants, bears, foxes, |
