OCR Text |
Show 468 GENERAL RESULTS. CHAP. XII. of sterility and the amount of ext~rnal difference ~n the sp cies which are crossed; and still mo;re clear~y 1n the wide difference in the results of eross1ng roc1procall y tho sam two species ;-that is, when species A is crossed with pollen from B, and then B is crossed with pollen from A. Bearing in mind what has just been t:mid on the extreme sensitiveness ancl delicate affinities of the reproductive syste1n, why should we fool any surprise at the sexual elements of those forms, which we call species, having been differentiated in such a 1nanner that they are incapable or only feebly capable of actin o· on one another? vV e know that species have b generally lived under the same conditions, and have retained their own proper characters, for a mueh longer period than varieties. Long-continued clon1estication eliminates, as I have shown in my 'Variation under Domestication,' the mutual sterility which distinct specjcs lately taken from a state of nature almost always exhibit when intercrossed; and we can thus understand th fact that the most different domestic races of ani1nals <:no not n1utunlly sterile. But whether this holds good with cultivated varieties of plants is not known, though son1e facts indicate that it does. The limination of sterility through long-continueJ don1cstication rr1ay probably Lo attributed to the varying conditions to which our don1estic animals have been suLjectcd; and no doubt it i owino· to this same cause that they withstand gr at and s:Cldon chang s in their condition~ of lifo with far less loss of fertility than do natural spec1es. Fron1 those several considerations it appears probable that tho difference in th affinities of the sexual el 'lllOnts of uistinct species, on which their 1nutual incapacity for breeding together depends, is caused.l>y thC'ir havino· been habituated for a v ry long penod c•·:~,eh to its ~wn conditions, and to the sexual el01ncnts CHAP. xn. GENERAL RE, ULTS. 469 ha_ving thus acquired firmly fixed affinities. II owovcr th1s may be, with the two great classes of cases b f rc us, namely, those relating to the self-fertilisation and eross-fertilisation of the individuals of the same sr ·i s, an~ those relating to the illegitimate and lcgitimat unions of heterostyled plants, it is quite unjustifiabl to assume that the sterility of species when first cro. s c1 a~d o~ their hybrid offspring, indicates that they differ 1n some fundainental1nanner from the vari eti s or individuals of the same species. |