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former are also susceptible of fertilisation by the male element of V. nigrum. The full explanation of these curious and complicated sexual relations, I leave for more sagacious and ingenious investigators, and simply confine myself to remarking on the apparent support that these and more especially those other cases which I have communicated to the Linnean Society,* on the fertilisation of certain species of Passifloræ, -in which I showed that individual plants perfectly self-sterile readily effected reciprocal unions with other similarly characterised individuals of the same species-give to that view which Mr. Darwin has propounded regarding the existence of a law in nature necessitating "an occasional cross with another individual, or, that no hermaphrodite fertilises itself for a perpetuity of generations," but "that some unknown great good is derived from the union of individuals which have been kept distinct for many generations."†

In the following table, the results of the pure unions of V. phoeniceum given on the first line are taken from capsules on a specimen in the Edinburgh University Herbarium, as I have not yet been successful in getting good capsules from any of the plants which I have had an opportunity to experiment upon by their own pollen. The other plants of V. phoeniceum and varieties mentioned in the table are the same as those from which I had the results given in Table 1. Indeed, in one or two instances, the same experiments are re-stated, with a view to show more clearly the relative degrees of sterility resulting from the crossing of undoubted varieties of a species on the one hand, with those from the hybridisation of distinct species on the other.

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In addition to the simple and calculated results given on Table 1, I have, in the above, given at the right hand, for the sake of comparison, the calculated product from an assumed 1,000 seeds of the pure unions relatively to those yielded by the cross and hybrid unions. By a further comparative study of these results, we find that the fertility of the pure unions of V. phoeniceum, relatively to that of its cross-unions with the white and rose-coloured varieties, is, in the least differentiated or most highly fertile unions, viz., V. phæniceum, rosea by pollen of V. phoeniceum, as 100: 95; whereas in the least fertile unions, V. phoeniceum by pollen of V. phoeniceum, alba, the proportions are as 100: 56. The average fertility of the five cross-unions given in the table, relatively to the pure unions given in the first line, is as 100: 75; so that the pure unions thus exceed in fertility the cross-unions, in nearly the proportions of 4: 3. Again

by a similar comparative study of the relative fertility of the pure unions of V. phoeniceum and the different hybrid unions given in the Table, we find that the highest degree of fertility results from the union. of V. ferrugineum (which perhaps is correctly regarded by De Candolle and others as a mere variety of V. phoeniceum) with V. phoeniceum, the proportions of the pure to the hybrid unions being as 100 59, in favour of the former. The lowest degree of fertility results from the unions of V. ovalifolium, with V. phoeniceum, the proportion of the pure to the hybrid-unions in this case being as 100 24.) Lastly the average fertility of the five hybrid unions given in the latter lines of the Table, relatively to the pure unions of V. phoeniceum, is nearly as 100: 40, or as 2.5 seeds of the pure unions to one of the hybrid unions. Thus, the relative differences in the degree of sterilisation resulting from the hybridisation of distinct species, and that from the cross-impregnation of varieties of a species, relatively in either case to the pure unions, is in the former as 2.5: 1, and in the latter as 4: 3.

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In Table 3 we have first the results of the pure unions of V. lychnitis, alba, and by comparing them with those resulting from fertilisation with the pollen of V. lychnitis, lutea, we find that the latter cross-unions undergo the proportionately decreased fertility of 100: 82. By the hybrid-unions of V. lychnitis, alba, with the pollen of V. phoniceum, alba, a slightly higher degree of sterilisation results; the proportion in this case being as 82: 67, relatively to 100 produced by the pure unions of V. lychnitis, alba. The highest degree of sterilisation in this Table results from the union of V. lychnitis, alba, by pollen of V. thapsus, alba, the proportion of the pure to the hybrid unions being here as 100: 47.

The results of my experiments on the yellow variety of V. lychnitis are given in Table 4. By a comparative examination of this Table, we have the following general results: first, the fertility of the pure unions of V. lychnitis, lutea exceeds that resulting from the cross-unions of the latter with pollen of V. lychnitis, alba, in the proportion of 100 94. The degree of sterilisation induced by these unions, though less than that resulting from the converse unions given in Table 3, is nevertheless sufficient to show a sterilising influence in the conjunctions of varieties of a species, characterised only by those, systematically considered, trifling differences in colour-the one being white, the other yellow. Secondly we have the results of unions of similarly and dissimilarly coloured forms of distinct species, with V. lychnitis, lutea. Thus the pollen of V. phoeniceum, with purplish coloured flowers, applied to the stigmas of V. lychnitis, lutea, gives an average fertility of 66; the pollen of the white variety V. phoeniceum, alba, gives an average of 55; while that of the rose-coloured variety is productive of the highest degree of sterilisation, giving only 49-relatively to 100, the produce of V. lychnitis, lutea by its own pollen. Mr. Darwin, on the authority of Gartner, states in his " Origin of Species," that similarly coloured varieties of distinct species are more fertile when crossed than are the dissimilarly coloured varieties of the same species. The particular illustration of this point will be found in a subsequent part of this paper; I will here merely state that, in the above unions, the degrees of fertility are by no means regulated by the colour affinities. Thus, we have first yellow and violet, then yellow and white, and lastly yellow and rose yielding a successively decreased fertility; whereas, judging by the colour affinities, the arrangement ought to have been, beginning with the most fertile, yellow first with white, then with rose, and lastly with violet. Secondly, with pollen of the V. blattaria, vars. alba and lutea, we see, that the V. lychnitis, lutea yields the higher degree of fertility with the former: V. lychnitis, lutea, yielding with pollen of V. blattaria, alba, 56, and with that of V. blattaria, lutea, 51, relatively to 100, the product of fertilisation with its own pollen. Thirdly, in the unions of V. lychnitis, lutea, by pollen of the yellow and white varieties of V. thapsus, we find that unions of the similarly coloured flowers are the more fertile. V. lychnitis, lutea, yielding with pollen of V. thapsus, lutea, 46, and with the pollen of V. thapsus, alba, 39, relatively to 100, .

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