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DARWIN ON THE EFFECTS OF CROSS AND SELF FERTILIZATION IN PLANTS.

It is impossible to finish the perusal of any of Mr. Charles Darwin's works without a genuine feeling of admiration, not only for the manner in which the investigator pursues every branch of the great principles he has enunciated to its minutest ramification, but for the almost inconceivable patience with which he accumulates grain after grain of proof, until his position is not only firmly established but seems possessed of even a superabundant support. For eleven years he has been conducting the difficult and delicate inquiry of which his recent volume is the record; and yet the result to be adduced, from the great mass of facts so slowly and laboriously gathered, is no strikingly novel discovery, although much is embodied that is new. It is rather a substantiation of opinions already enunciated, leading to their wider generalization. His conclusion is closely connected " with various important physiological problems, such as the benefit derived from slight changes in the conditions of life, and this stands in the closest connection with life itself. It throws light on the origin of the two sexes, and on their separation or union in the same individual, and lastly on the whole subject of hybridism, which is one of the greatest obstacles to the general acceptance and progress of the great principle of evolution."

In briefly reviewing Mr. Darwin's new work, or rather its conclusions, for we cannot attempt the consideration of his countless experiments, it is best to begin by the repetition of his own statement, made to avoid misapprehension, namely, that the term "crossed plant seedling, or seed," means one of crossed parentage, that is, one derived from a flower fertilized with pollen from a distinct plant of the same species. And a self-fertilized plant seedling or seed means one of self-fertilized parentage, that is, one derived from a flower fertilized with pollen from the same flower, or sometimes from another flower on the same plant.

From his observations on plants, and guided to a certain extent by the experience of breeders of animals, Mr. Darwin many years ago became convinced that it is a general law of Nature that flowers are adapted to be crossed at least occasionally by pollen from a distinct plant. It often occurred to him that it would be advisable to try whether seedlings from cross-fertilized flowers were in any way superior to those from self-fertilized flowers. It so happened that, without any thought of the above inquiry, he raised close together two large beds of self-fertilized and crossed seedlings from the same plant of linaria vulgaris. To his surprise, the crossed plants, when fully grown, were plainly taller and more vigorous than the self-fertilized ones. As it seemed quite incredible that the difference between the two beds of seedlings could have been due to a single act of self-fertilization, Mr. Darwin attributed the fact to some accidental cause ; but in order to test the matter, he  prepared two more beds from the carnation dianthus caryophyllus, which, like the linaria is almost sterile when insects are excluded; and, hence the inference may be drawn that the parent plants must have been intercrossed during every, or almost every, previous generation. Nevertheless, the self-fertilized seedlings were plainly inferior in height and vigor to the others. This was the starting point of Mr. Darwin's experiments, conducted with all the refinement and minuteness necessary for the most accurate of observations.

Of the conclusions reached, the first and most important is that cross-fertilization is generally beneficial, and self-fertilization injurious. This is shown by the difference in height, weight, constitutional vigor, and fertility of the offspring from crossed and self-fertilized flowers, and in the number of seeds produced by the parent plants. The advantages of cross-fertilization do not follow from some mysterious virtue in the mere union of two distinct individuals, but from such individuals having been subjected during previous generations to different conditions, or to their having varied in a manner commonly called spontaneous; so that in either case their sexual elements have in some degree differentiated.

Again, the injury from self-fertilization follows from the want of such differentiation in the sexual elements. Thus when plants of the ipomœa and of the mimulus, which had been self-fertilized for the seven previous generations, and had been kept all the time under the same conditions, were intercrossed one with another, the offspring did not profit in the least by the cross. On the other hand, as showing that the benefit of a cross depends on the previous treatment of the progenitors, plants which had been self-fertilized for the eight previous generations were crossed with plants which had been intercrossed for the same number of generations, all having been kept under the same conditions as far as possible. Seedlings from this cross were grown in competition with others derived from the self-fertilized mother-plant crossed by a fresh stock; and the latter seed­ lings were to the former in height as 100 to 52, and in fertility as 100 to 4. Under a practical point of view, agriculturalists and horticulturists may learn much from the above conclusions. Thus it appears that the injury from the close breeding of animals and from the self-fertilization of plants does not necessarily depend on any tendency to disease or weakness common to the constitution of the related parents, an only indirectly on their relationship, in so far are as they are apt to resemble each other in all respects, including their sexual nature ; and secondly, that the advantages of cross-fertilization depend on the sexual elements of the parents having become in some degree differentiated by the exposure of their progenitors to different conditions, or from their having intercrossed with individuals thus exposed, or from spontaneous variation. Animals to be paired should therefore be kept under as different conditions as possible, and excellent results have been obtained from the interbreeding of individuals reared on distant and differently situated farms. With all species of plants which freely intercross, by the aid of insects or the wind, the best plan is to secure seeds of the required variety which have been raised for some generations under as different conditions as possible, and sow them in alternate rows with seeds matured in the old garden. The intercrossing of the stocks will yield far more favorable results than any mere exchange of seeds. Florists may learn that they have the power of fixing each fleeting variety of color, if they will fertilize the flowers of the desired kind with their own pollen for half a dozen generations, and from the seedlings under the same conditions. But a cross with any other individual of the same variety must be carefully prevented, as each has its own constitution. After a dozen generations of self-fertilization, the new variety will probably remain constant, even if grown under different conditions; and there is no longer any necessity of guarding against intercrossing.

With respect to mankind, Mr. George Darwin has concluded, from a statistical investigation which has already been reviewed in these columns, that the evidence of any evil due to the intermarriage of first cousins is conflicting, and on the whole points to the same being very small. Our author infers that, with mankind, the marriages of nearly related persons, some of whose parents and ancestors had lived under very different conditions, would be much less injurious than that of persons who had always lived in the same place and followed the same habits of life. He sees no reason to doubt that the widely different habits of life of men and women in civilized nations, especially amongst the upper classes, would tend to counterbalance any evil from marriages between healthy and somewhat closely related persons.