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Show 168 DARWINISM CliAl'. in those found in the same area but frequenting somewhat different stations. To carry out these experiments with any satisfactory result, it will be necessary to avoid the evil effects of confinement and of too close interbreeding. If birds arc experimented with, they should be allowed as much lib?rty as pos iblc? a plot of ground with trees and bushes bcmg enclosed ~1th wire nettincr overhead so as to form a large open avJary. The spccies 0 experimented with should be obtn.ined in c?nsiderable numbers, and by two separate persons, each ma.kmg the opposite reciprocal cross, as explained at p. 155. In the second crencration these two stocks might be themselves crossed to prev~nt the evil effects of too close interbreeding. By such experiments, carefully carried out with different groups of animals ancl plants, we should obtain a body of facts of a character now sadly wanting, and without which it is hopel ess to expect to arrive at a complete solution of this difficult problem. There arc, however, some other aspects of the question that need to be consiclcrcd, and some theoretical views which require to be carefully examined, ha.ving done which we shall be in a condition to state the general conclusions to which the facts and reasonings at our command seem to point. Sterility due to changed Conditions and 1ts1wlly con·elated with other Characte1·s, especially with Colou1·. The evidence already adduced as to the extreme su ccptibility of the rcproducti vc system, and the curious in·cgnlarity with which infertility or sterility n.ppcars in the crosses between some varieties or species while quite absent in th ose between others, seem to indicate that sterility is a chamctcristic which has a constant tendency to appear, either hy itself or in correlation with other character·. It is kn own to he especially liable to occur under changccl collditiom; of life; and, as such change is usually tho stitrtiJJ ~'-point HI Hl cause of the development of new species, we h<LVC alrea,dy found a reason why it should so often appear when species become fully differentiated. In almost all the cases of infertility or sterility between varieties or species, we have some external differences with VI! ON TilE INFERTILITY OF CROSSES 169 which it is correlated ; and though these cliffcre.wcs arc sometimes slight, and the amount of the infertility is not always, or even usually, proportimmtc to tho external difference between the two forms crossed, we must believe t,hat there is omc connection between the two classe. of facts. This is c pccially the case as regards colour ; and Mr. Darwin has collected a body of facts which go far to provo that · colour, instead of being an 1dtoo·cthcr trifling :tnd unimportant character, as was supposed by the older naturalists, is really one of grca.t significance, sin ·e it is undoubtedly often correlated with important constitutionn,l differences. Now colour is one of the characters that most usua.lly distinguishes clo. ely allied species; and when we hear that the most closely allied species of plants are infertile together, while tho. c more remote are fertile, the meaning usually is that the form r differ chiefly in the colom· of their flowers, while the latter differ in the form of the flowers or foliage, in habit, or in other structural characters. It is therefore a most curious and suggestive fact, that in all the recorded ca cs, in which a d •ciclcd infertility occurs between varieties of the same species, those varieties arc distinguished by a difference of colour. The infertile varieties of V crbascum were white and yellow flowered respectively; the infertile varieties of m<tize were red and yellow seeded; while the infertile pimpernels were the red and the blue flowered varictic . So, the differently coloured varieties of hollyhocks, though grown close together, each reproduce their own colour from seed, showing ,that they arc not capable of freely intercrossing. Yet Mr. Darwin assnrcs us that the <Lgcncy of bees is nccessn.ry to cany the pollen from one plant to another, bccau. c in each flower the pollen is shed before the stigma is ready to receive it. \V c have here, therefore, either almo t complete sterility between vari •ti ·. of different colours, or a prepotent effect of pollen from a flower of the same eolour, bringing about the same result. Similar phcnomcnit have not been rccordc(l :tmona animals; but this is not to he wondered n.t when we conside~ that most of our pure a,ll(l vaJucd domestic breeds arc characterised by definite eolonrs which constitute one of their |
