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Show 442 GENERAL RESULTS. CHAP. XII. Moreover, the advantages which follow fro~ cros~fertilisation differ 1nuch in different plants, so that It is probable that allied plants. :vot:ld often profit in different deo-rees by cross-fertlhsatlon. Under those extremely c~n1plex and fluctuating. conditions, with two somewhat opposed ends to be gained, namel_Y, the safe propagation of the species. and. t~o productio:l. of cross-fertilised, vigorous offspnng, It IS not surpnsing that allied forn1s should exhibit an extreme eli vcrsity in the means which favour either end. If, as there is reason to suspect, self-fertilisation is in some respects .beneficial, although n1ore than counterbalanced. by the advantages derived from a cross with a fresh stock, the problem becomes still more complicated. . As I only twice exporimentou on more than a single species in a genus, I cannot. say .wh_ethor tho crossed offsprino- of the several species witlun the same genus differ i~ their degree of superiority ov r their sel~fertilised brethren; but I should expect that this would often prove to be the case fro1n what was observed with the two species of Lobelia and with the individuals of the same spoci s of Nicotiana. The species belonging to distinct genera in the same family certainly differ in this resp ct. Tho offe?ts of crossand self-fertilisation may be con£nod either to the growth or to the fertility of the of-t pring, but generally extends to both qualities. Th re does not seen1 to · exist any close correspondence b tween the ~ogree to which the flowers of speci s are adapted for cr~ssfertilisation, and the degree to which their · offspnn.g profit by this process; but we may eas.ily err on t~~s head as there are two m ans f r ensunng cross-feitllisation which are not ext rnally p rcoptible, namely, self-sterility and the prepot nt f rtilising influence of pollen fron1 another individual. Lastly, it has been CIIAP. XII. GENERAL RESULTS . 443 shown in a former chai)ter that th .C(' • . d . . . c e euoct produc l b cross an self-fertilisation on th f ·t·l· . e Y 1 t d e or l Ity of the par nt-p an s oes not always correspond with th· t cl on the height, vigour, and fertility of the~lr' p~o l:ced Th . 1. . ouspnnO' e ~arne romar \: apphos to crossed and self-fertilisecl seedhngs when these are lised as the paren t -p l ants. This want of correspondence probably depends at 1 t . ·t th · , eas In par ' on e number of seeds produced be· h. fi d . 1ng c Ie y etermrned by the num bor of the po 11 en-t u b es wh 'I Ch rea?h the ovules, and this will be governed by the re-actr? n between the pollen and the stigmatic secretion ofr trhs sues; w. .h ereas. the gro.w th and constr't u t.r ona l vi.g our o t o offspnng Will be chre:fly determined, not only by the number of pollen-tubes reaching the ovules but b the nature. of the reaction between the contents of th~ pollen-grarns and ovules. There are tw? other important conclusions which may be deduced ±rOJn my observations: firstly, that the advantages o.f cross-fertilisation do not follow from s~m~ m!st~r~ous virtue in the mere union of two d1strnct r~drvrduals, .but from. such individuals having been subJe?~ecl dunng prevrous generations to different condrtrons, or to their having varied in a 1nanner co~monly called spontaneous, so that in either case the~r sexual elements have been in some degree differ~ entr.a~ed.. And secondly, that the injury from selff~ rtrl~satron follows from the want of such differentiatr~ n In the sex~al elem nts. These two propositions are .. fully established by my expori1nents. Thus, when plants of the Ipomma and of tho Mimulus, which had been self-fertilised for the seven previous generations a~d had been kept all the tin1e under the same condi~ l.ons, were inter?rossed one with another, the offspring Id not profit 111 the least by the cross. Mimulus |
