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Show 450 GENERAL RESULTS. CHAP. XII. or ob lI. te rat 1• 0n of some of these characters, and. to the rea ppearan Ce of forn1er ones through rov.o rs1on; and so it will be, as we may feel almost sure, .with any slight differences in the consti~ntion o~ tl:ou soxnal e1 e ment s. A n Yhow , my eXl)enments ln<hcn. ,te that crossing plants which have been longd.s~1bJ oct?d hto a1m ost tho ubo· h not quite tho same con Itl.o.n s, IS t o mos t power ful of a.l l .t he .m oans for retaining some cleo-reo of differentiation In tho sexual l~monts, as sh~wn by the superiority in tho later gen~ratlons of tho intercrossed over the self-fertilised soeclhngs. N over-the 1 e ss, the continued intorcrossing .o f plan. ts. thus treated docs tend to obliterate such differen~1at1on, as be inferred from tho lessoned benefit den ved from may · · h h f intercrossing such plants, in companson \\It t at rom a cross with a fresh stock. It se ms probable, as I ~ay add that seeds have acquired their endless cunous ada~tations for wide dissemination,* not only that ~he seedlings should thus bo enabled to .find new and fitting homes, but that the individuals w luch have beo~1 long subjected to the saine conditions should occasionally intercross with a fresh stock. From the foregoing several considerations w.o .rna!, I think conclude that in tho abov case of tho Digitalis, and ev~n in that of plants which have grown for thousands of generations in the .same di~trict, as 1nust often have occurred with spec1os having a 1nuch restricted range, we aro apt to over-estimat~ the deoTee to which the indivi<luals have been suLJoctecl to babsolutely the sn,me conditions. There is at least no difficulty in believing that such plan1 s have ?oen subjected to sufficiently distinct con dI. ti·o ns t o d1 f for - entiate their sexual clements ; for we know that a plant * -- ~V ·b ·eitun(fsroittel der See Prof. Hild.ebrand's excellent treatise, er I o Pilauzen,' 1873. CIIAP. XII. GENERAL RESULTS. 451 propagatecl for some generations in another , ·d . th d. · · ga.r n 1n e same Istnct serves as a fresh stocl ... a d h· h. h .[! .1. . ~ c n as l ()' 1ertl ISin.g . powers. The, curious cases of p 1a nts wh .1b h can fertilise and. be fertilised by any oth er I. na ·I VI· au a l of the saino speCies, but arc altogether sterile with their own poll~n, become intelligible, if the view here propounded I~ correct; namely, that the individuals of the same spec1es growing in a state of nature near to()'ether have no.t really L~en subjected during several p;eviou~ generations to quite the same conditions. Soine n~turalists assume that there is an innate tende~cy. 1n .all beings to vary and to advance in organisation, Independently of external agencies; ancl they would, I presume, thus explain the slio·ht differences which distinguish all tho individuals of ~he same species both in external characters and in constitution, as well as the greater differences in both respects between nearly allied varieties. N 0 two individuals can be found quite alike; thus if we sow a numbe1~ of seeds from the same capsule under as nearly a~ possible the same conditions, they germinate at di~erent rates and grow more or less vigorously. They resist cold and other unfavourable conditions differently. They would in all probability, as we know to be the c~se with ani1nals of the same species, be somewhat d~fferently acted on by the same poison, or by the same disease. They have different powers* of transmittino- ' their characters to their offspring; and many analogou~ facts could be given. Now, if it were true that plants growing ncar together in a state of nature had been subjected during many previous generations to absolutely the Saine conditions, such differences as those iust specified would be quite inexplicable; but they "' Vilmurin, as quoted by Verlot, 'De>3 Varieies,' pp. 32, 38, 39. 2 G 2 .• |
