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Show 456 GENERAL RESULTS. CHAP. XII. pollen from another flower on the same plant, they are sometimes, though rarely, a little more fertile; if fertilised with pollen from another individual or variety of the same species, they are fully fertile ; but if with pollen from a distinct species, they arc storil13 in all possible degrees, until utter sterility is reached. We thus have a long series with absolute sterility at the two ends ;-at one end due to the sexual clements not having been sufficiently differentiated, and at the other end to their having been differentiated in too great a degree, or in some peculiar manner. The fertilisation of one of the higher plants depends, in the first place, on the mutual action of the pollengrains and the stigmatic secretion or tissues, and afterwards on the mutual action of the contents of the pollen-grains and oYules. Both actions, judging from the increased fertility of the l)arent-plants and from the increased powers of growth in the offspring, are favoured by some degree of differentiation in the elements which interact and unite so as to form a new being. Here we hav some analogy with chemical affinity or attraction, which comes into play only between atoms or molecules of a different nature. .lis Prof. lVIiller remarks: " Generally speaking, the greater the difference in the properties of two bodies, the more intense is their tendency to mutual chemical action. . .. But between bodies of a similar character the tendency to unite is feeble."* This latt r proposition accords well with the feeble effi cts of a plant's own pollen on the fertility of the mother-plant and on the growth of the offspring; and the form r proposition accords well with the powerful influence in both ways of pollen from an * 'Elements of Chemistry,' 4th edit. 18G7, part i. p. 11. Dr. ·Frankland informs me that similar views with respect to chemical affinity nre generally accepted Ly clwmi:::.t::;. UHAP. XII. GENERAL RESULTS. 457 individual which has been differentiated by exposur to .ch~nged conditions, or by so-called spontan ous vanat~on. But the analogy fails when we turn to the ne~at: ve or w~ak effects of pollen from one species on a distinct species ; for although some substances which are extremely dissimilar, for instance, car bon and chlo~·ine, have a very feeble affinity for each other, yet It ca~not be said that the weakness of the affinity depends In such cases on the extent to which the substances ~iffer. It is not known why a certain ainount of differentiation is necessary or favourable for the chemical affinity or union of two substances, any more than for the fertilisation or union of two organisms. Mr. Herbert Spencer has discussed this whole subject at great length, and after stating that all the forces throughout nature tend towards an equilibrium, remarks, "that the need of this union of spenncell and germ -cell is the need for overthrowing this equilibrium and re-establishl.ng active molecular change in the detached germ --a result which is probably effected by mixing the slightly-different physiological units of slightly-different individuals."* But we must not allow this highly generalised view, or the analogy of chemical affinity, to conceal from us our ignorance. We do not know what is the nature or degree of the differentiation in the sexual elements which is favourable for union, and what is injurious for union, as in the case of distinct species. We cannot * 'Principles of Biology,' vnl. i. p. 274, 1864. In my 'Origin of Species,' published in 1859, I s~oke of the good effects from slight changes in the condition of life and from cross-fertilisation aud of the evil effects from great changes in the conditions and · from crossing widely distinct forms (i.e., species), as a series of fact::; " connected together by some common but unknown bond, which is essentially related to the principle of life." |