Self-fertilization

“ In fact, there can be no doubt that it is in order to insure this cross-fertilization that the sperm-particles are so light and abundant; and the final cause of this, again, is that cross-fertilization produces better results than self-fertilization. But if so, then Nature will take steps not only to insure cross-fertilization, but to prevent self-fertilization. This in animals as in plants can only be done in two ways, viz., either by so placing the two organs that self-fertilization is impossible, or else by separating them in different individuals. ”

“ Having remarked that "self-fertilization would have been an incomparably safer and easier process than the transportal of pollen from flower to flower," he adds these words,—"It is hardly an exaggeration to say that Nature tells us, in the most emphatic manner, that she abhors perpetual self-fertilization." ”

“ He recognized that, in general, self-fertilization is disadvantageous to plants; that they produce fewer seeds, and that these produce feebler plants, than when they are cross-fertilized; that, therefore, those flowers which are arranged to secure cross-fertilization have an advantage over those which are self-fertilized. In many species, as Sprengel had already pointed out, self-fertilization leads to actual infertility; only a few plants are as fertile with their own pollen as with that of another plant ”

“ Darwin has shown that, although capable of self-fertilization, yet cross-fertilization—i. e., the fertilization of the ovules of one flower, or, still better, of the flowers of one plant by the pollen of another—produces more seeds, larger seeds, and stronger seedlings; in other words, produces better results. Now, it is a law which necessarily results from the principle of the survival of the fittest that Nature ever strives to secure better results. Therefore, she immediately sets to work to contrive methods of insuring cross-fertilization and preventing self-fertilization. ”

“ If, then, self-fertilization had to be guarded against or at least rendered difficult because its consequences were injurious, all variations pointing in the direction of safeguarding would necessarily be preserved and increased. ”

“ Self-fertilization in its strictest sense occurs within the individual flower. Plants only resort to it as an extreme measure and commonly make use of many devices to prevent it. In the Iris, the petal-like stamens are in direct contact with the pistil and yet self-fertilization does not result, because the pollen surface is always carefully turned away from the ovary. ”

“ Among domestic animals close interbreeding, it is well known, leads to infertility and degeneration; and Darwin's researches prove that self-fertilization in the vegetable kingdom produces the same results. ”

“ So general is this cross-fertilization, so much a part of the economy of nature does it appear to be, that we can only think that there must be in the production of this vast horde of the cross-fertilized some advantage. Besides securing greater virility, which almost certainly results from this promiscuity, greater variability is promoted. If virility is the result, the price paid for it is tremendous, for the hindrances to self-fertilization are so many and so effective that most flowers would inevitably die as perfectly pure but ineffectual virgins if that fatality were not prevented. ”

“ But cases of exclusive and constant self-fertilization do also seem to occur, and even these may be brought into harmony with our conception, according to which cross-fertilization is an advantage, but only an advantage which must be weighed against others, and which may eventually be given up in favour of greater advantages. ”

“ That the injury from self-fertilization follows from the want of such differentiation in the sexual elements. These two propositions are fully established by my experiments. 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. ”

“ In the flower where the father and mother part matures at the same time, self-fertilization is the rule. Cross-fertilization occurs in instances where either the father part or mother part ripen at different times, in these cases the pollen is carried from plant to plant by the wind or by the nectar-seeking bees. ”

“ Darwin, by his careful studies on the effects of cross- and self-fertilization in the vegetable kingdom, contributed more largely than any one else to the discovery of this law. He watched, from germination to maturity, more than a thousand individual plants, produced by crossing and self-fertilization, belonging to fifty-seven species, fifty-two genera, and thirty large families, and including natives of the most various countries.1980 The result established by this research was, that cross-fertilization is generally beneficial, and self-fertilization injurious ”

“ It is probable that in many cases the opposite or retrograde change occurred, and that the difficulties in the way of cross-fertilization compelled a return to self-fertilizing bisexuality. Such retrograde changes are common in evolution. ”

“ We are not therefore justified in admitting that this peculiar state of the reproductive system has been gradually acquired through natural selection; but we must look at it as an incidental result, dependent on the conditions to which the plants have been subjected, like the ordinary sterility caused in the case of animals by confinement, and in the case of plants by too much manure, heat, etc. I do not, however, wish to maintain that self-sterility may not sometimes be of service to a plant in preventing self-fertilisation ”

“ In the first place, there is no close correspondence in degree between the sterility of the parent-plants when self-fertilised, and the extent to which their offspring suffer in vigour by this process; and some such correspondence might have been expected if self-sterility had been acquired on account of the injury caused by self-fertilisation. The fact of individuals of the same parentage differing greatly in their degree of self-sterility is likewise opposed to such a belief ”

“ In the chapter on Fertility, when we consider the effects of cross-fertilisation and self-fertilisation on the productiveness of the parent-plants we shall arrive at nearly the same result, namely, that a cross between the flowers on the same plant does not at all increase the number of the seeds, or only occasionally and to a slight degree. ”

“ Yet more extraordinary is it to find numerous cases in which the special arrangements for cross-fertilisation appear to have been a failure, since they have either been supplemented by special means for self-fertilisation, or have reverted back in various degrees to simpler forms in which self-fertilisation becomes the rule. ”

“ Fecundal selection is said by them to be constantly tending to increase the reproductive rate, because fecundity is partly a matter of heredity, and the fecund parents leave more offspring with the same characteristic. Lethal selection, on the contrary, constantly asserts its power to reduce the reproductive rate, because the reproductive demands on the parents reduce their chances of life by interference with their natural ability of self-protection. ”

“ Under these extremely complex and fluctuating conditions, with two somewhat opposed ends to be gained, namely, the safe propagation of the species and the production of cross-fertilised, vigorous offspring, it is not surprising that allied forms should exhibit an extreme diversity in the means which favour either end. If, as there is reason to suspect, self-fertilisation is in some respects beneficial, although more than counterbalanced by the advantages derived from a cross with a fresh stock, the problem becomes still more complicated. ”

“ We are therefore driven to the suspicion, which we shall soon see strengthened, that Hero transmitted to its offspring a peculiar constitution adapted for self-fertilisation.
It would appear that the self-fertilised descendants of Hero have not only inherited from Hero a power of growth equal to that of the ordinary intercrossed plants, but have become more fertile when self-fertilised than is usual with the plants of the present species. ”

“ That certain plants, for instance, Viola tricolor, Digitalis purpurea, Sarothamnus scoparius, Cyclamen persicum, etc., which have been naturally cross-fertilised for many or all previous generations, should suffer to an extreme degree from a single act of self-fertilisation is a most surprising fact. Nothing of the kkind has been observed in our domestic animals ”

“ Before proceeding any further, the view which has been maintained by several physiologists must be noticed, namely, that all the evils from breeding animals too closely, and no doubt, as they would say, from the self-fertilisation of plants, is the result of the increase of some morbid tendency or weakness of constitution common to the closely related parents, or to the two sexes of hermaphrodite plants. ”

“ If the conclusion at which he ultimately arrived, that cross-fertilisation is favourable to the fertility of the parent and to the vigour of the offspring, is correct, then it follows that all those mechanisms which hinder self-fertilisation and favour crossing [297] must be advantageous in the struggle for existence; and, the more perfect the action of the mechanism, the greater the advantage. Thus the way lay open for the operation of natural selection in gradually perfecting the flower as a fertilisation-trap. Analogous reasoning applies to the fertilising insect. ”

“ The facts as to the cross-fertilisation of flowers which have now been very briefly summarised, taken in connection with Darwin's experiments proving the increased vigour and fertility given by cross-fertilisation, seem amply to justify his aphorism that "Nature abhors self-fertilisation," and his more precise statement, that, "No plant is perpetually self-fertilised;" ”

“ Of such cases there are twelve in the table, and the mean of their mean fertility is as 100 for the crossed plants, to 59 for the self-fertilised plants. The Primulaceae seem eminently liable to suffer in fertility from self-fertilisation. ”

“ It has been shown in the present volume that the offspring from the union of two distinct individuals, especially if their progenitors have been subjected to very different conditions, have an immense advantage in height, weight, constitutional vigour and fertility over the self-fertilised offspring from one of the same parents. ”

“ In some instances the sexes may have been re-separated for the sake of preventing too frequent self-fertilisation; but this explanation does not seem probable, as the same end might have been gained by other and simpler means, for instance dichogamy. It may be that the production of the male and female reproductive elements and the maturation of the ovules was too great a strain and expenditure of vital force for a single individual to withstand, if endowed with a highly complex organisation ”

“ Notwithstanding the several considerations just advanced, it seems to me highly improbable that plants bearing small and inconspicuous flowers have been or should continue to be subjected to self-fertilisation for a long series of generations. I think so, not from the evil which manifestly follows from self-fertilisation, in many cases even in the first generation, as with Viola tricolor, Sarothamnus, Nemophila, Cyclamen, etc. ”

“ It has, however, always to be remembered that as we rise in the zoölogical scale, and as the period of gestation lengthens and the possible number of offspring is fewer, it becomes constantly more essential that fertilization shall be effective rather than easy; the fewer the progeny the more necessary it is that they shall be vigorous enough to survive. ”

“ The manner in which, in the human species, fertilization is effected, is still far from clear in all its details; hence it is easy to understand, that the etiology of sterility remains in many respects obscure. It is impossible in every case to find a definite cause. Whereas, on the one hand, notwithstanding the existence of apparently insuperable obstacles, impregnation may nevertheless be effected; so, on the other hand, sterility may exist in cases in which all the circumstances appear favourable to the occurrence of conception. ”

“ Consequently if two plants which thus differ are crossed, the seedlings suffer and are beaten by those from the self-fertilised flowers, in which the sexual elements are of the same nature. It is known that with our domestic animals certain individuals are sexually incompatible, and will not produce offspring, although fertile with other individuals. ”