OCR Text |
Show Hl2 DARWINISM OIIAP. form or colour, or with inherent peculiarities of likes or dislikes lcading_to any choice as to the pairing of the two sctR of individuals. We have now to inquire, What would be the result? Taking, first, the 10,000 pairs of the physiological or abnormal variety, we find tha.t each male of these might pair with any one of the whole 100,000 of the opposite sex. If, therefore, there was nothing to limit their choice to particular individuals of either variety, the probabilities are that 9000 of them would pair with the opposite variety, and only 1000 with their own variety-that is, that 9000 would form sterile unions, and only one thousand would form fertile unions. Taking, next, the 90,000 normal individuals of either sex, we find, that each male of these has also a choice of 100,000 to pair with. The probabilities arc, therefore, that nin etenths of them-that i., 81,000-would pair with their normal fellows, while 9000 would pair with the opposite abnormal variety forming the above-mentioned sterile 11nions. Now, as the number of individuals forming a species remains constant, generally spca.king, from year to year, we shall have next year also 100,000 pairs, of which the two physiological va.rieties will be in the proportion of eighty-one to one, or 9 , 780 pairs of the normal variety to 1220 1 of the abnormal, that being the proportion of the fertile unions of each. In this year we shall find, by the same rule of probabilities, tha.t only 15 :males of the abnormal vn.riety ·will pa.ir with their like and be fertile, the remaining 1205 forming sterile unions with some of the normal v:tricty. The follow ing year the total 100,000 pairs will consist of 99,984 of the normal, and only 16 of the a.bnormal variety; and the pro], :Lbilitics, of course, arc, that the whole of these latter wi ll pair with some of the enormous preponderance of normal individuals, and, their unions being sterile, the physiological variety will become extinct in the third year. If now in the second and each succeeding year a s;imilnr proportion as at first (1 0 per cent) of the physiological variety is produced afresh from the ranks of the norma.l vn.ricty, the same rate of diminution will go on, and it will be found that, 1 The exact number is 1219 ·51, but the fractions n:re omitted for clefl.mess. VII ON THE INFERTILITY OF CROSSES 183 on the most favourable estimate, the physiological variety can never exceed 12,000 to the H8,000 of the normal form of the species, as shown by the following table:- 1st Year. 2d 3d 10,000 of physiological variety to 90,000 of 11ormal variety. 1,220 + 10,000 again produced. 16 + 1,220 + 10,000 do. = 11,236 4th 0 + 16 + 1, 220 + 10,000 do. = 11,236 5th , 0 + 16 + 1,220 + 10,000 = 11,236 and so on for any number of generations. In the preceding discussion we ha vc given the theory the advantage of tho large proportion ·of 10 per cent of this very exceptional variety arising in its midst year by yea.r, an!l we have seen that, even under these fu.vountble conditjons, it is una.ble to increase its numbers much a.bovc its starting-point, and that it remains wholly dependent on the continued renewal of the variety for its existence beyond a few years. It appea.rs, then, that this form of inter -specific sterility cannot be increased by n:ttural or a,ny other known form of selection, but that it conta.ins within itself its own principle of destruction. If it is proposed to get over the difficulty by postulating a larger percentage of the variety annually a.rising within the species, we shall not affect the law of decrease until we approach cqua.lity in the numbers of the two varieties. But with any such increase of the physiological variety tho species itself would inevitably suffer by the large proportion of sterile unions in its midst, and wonld thns be a.t a great disadva.ntagc in competition with other . pccies which were fertile throughout. Thus, lJa.tnml selection will always tend to weed out a.ny species with too great a tendency to sterility among its own members, a.nd will therefore prevent snch sterility from becoming the general characteristic of varying species, which thi theory demands should be the ca 'C. On the whole, then, it appc:trs clear that no form of iufcrtility or sterility between the individuals of a species, can be increased by natural selection unless correla,ted with some useful variation, while all infertility not so correlated has a constant tendency to effect its own elimination. Rut the opposite property, fertility, is of vital importance to every species, and gives the offspring of the individuals which possess it, in consequence of their superior numbers, a greater |