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Show 452 GENERAL RESULTS. CHAP. XII. are to a certain extent intelligible in accordance with the views just advanced. As most of the plants on which I experimented were grown in my garden or in pots under glass, a few words must be added on the conditions to which they were exposed, as well as on the effects of culti_vation. vVhen a species is first brought under culture, It may or may not be subjected to a chan~e of climate, but it is always grown in ground broken up, and more or less manured ; it is also saved from competition with other plants. The· paramount importance of this latter circumstance is proved by the multitude of species which flourish and multiply in a garden, but cannot exist unless they are protected from other plants. When thus saved from competition they are able to get whatever they require from the soil, probably often in excess; and they are thus subjected to a great change of conditions. It is probably in chief part owing to this cause that all plants with rare exceptions vary after being cultivat d for some generations. The individuals which have alr ady begun to vary will intercross one with an ther by the aid of insects; and this accounts for the extreme diversity of character which many of our long cultivated plants exhibit. But it should be observed that the result will be largely determined by the degree of their variability and by the frequency of the intercrosses; for if a plant varies very little, like m st sp cies in a state of nature,. frequent intercrosses tend to give uniformity of character to it. I have attempted to show that with plants growing naturally in the same district, except in the unusual case of each individual being surrounded by exac.tly the same proportional numbers of other speci~s havmg certain powers of absorption, each will be subJected to CHAP. XII. GENERAL RESULTS. 453 slightly different conditions. Th' d the individuals of the same spec· Is :es not apply to cleared ground i th Ies w en cultivated in . . n e same garden. But if their ftlho.w ers 'al re V. ISited by insects ' the y WI' ll I. ntercross . and IS ·aW I .I give to their sexual e1 e ments dun.n ' g a consi erable number of generations a ffi . f d ' .Ci! • • su Cient amount o rueredn tiation for a cross 'to be b ene fi c1. a l . M ore-over, see s are fre~ uen tl y exchanged or procured f other gardens having a different kind f 'I· d rom . d' 'd l 0 SOl ' an the In IVI ua s of the same cultivated s . '11 h b . pec1es WI t us be su ~ec.t e.d to a change of conditions · If' th e fl owers arp not . VISited by our native insects , or very rareI y so as In the case of the common and sweet l' tl . h pea, anc apparen y In t at of the tobacco when kept . . hothouse, an. y differentiation in the sexua l e1 e mI enn t<s.t caused by Intercrosses will tend to disappear. This appe~rs to have occurred with the plants just mentioned, fo~· they were not benefited by bein crossed one with ano~her, though they were great!; bene:fi ted by a cross WIth a fresh stock. I have been led to the views just advanced with respect to the causes of the differentiation of the sexual element~ and of the varia?ility of our garden plants, by t~e results of my vanous experiments, and more espeCially ?Y the four cases in which extremely inconstant species, after having been self-fertilised and grown .under closely similar conditions for several ~enerations, produced flowers of a uniform and constant tint .. These conditions were nearly the same as those to which plants, growing in a garden clear of weeds are subjected, if they are propagated by self-fertilised seeds on · the same spot. The plants in pots were, however, exposed to less severe fluctuations of climate than those. out of doors; but their conditions, though closely uniform . for all the individuals of the same |
