III. We should expect to find a predominance of sexual over asexual generation, in the arrangements of nature for continuing her various species, inasmuch as two heads are better than one, and a locus pœnitentiæ is thus given to the embryo—an opportunity of correcting the experience of one parent by that of the other. And this is what the more intelligent embryos may be supposed to do; for there would seem little reason to doubt that there are clever embryos and stupid embryos, with better or worse memories, as the case may be, of how they dealt with their protoplasm before, and better or worse able to see how they can do better now; and that embryos differ as widely in intellectual and moral capacity, and in a general sense of the fitness of things, and of what will look well into the bargain, as those larger embryos—to wit, children—do. Indeed it would seem probable that all our mental powers must go through a quasi-embryological condition, much as the power of keeping, and wisely spending, money must do so, and that all the qualities of human thought and character are to be found in the embryo.
Those who have observed at what an early age differences of intellect and temper show themselves in the young, for example, of cats and dogs, will find it difficult to doubt that from the very moment of impregnation, and onward, there has been a corresponding difference in the embryo—and that of six unborn puppies, one, we will say, has been throughout the whole process of development more sensible and better looking—a nicer embryo, in fact—than the others.
IV. We should expect to find that all species, whether of plants or animals, are occasionally benefited by a cross; but we should also expect that a cross should have a tendency to introduce a disturbing element, if it be too wide, inasmuch as the offspring would be pulled hither and thither by two conflicting memories or advices, much as though a number of people speaking at once were without previous warning to advise an unhappy performer to vary his ordinary performance—one set of people telling him he has always hitherto done thus, and the other saying no less loudly that he did it thus;—and he were suddenly to become convinced that they each spoke the truth. In such a case he will either completely break down, if the advice be too conflicting, or if it be less conflicting, he may yet be so exhausted by the one supreme effort of fusing these experiences that he will never be able to perform again; or if the conflict of experience be not great enough to produce such a permanent effect as this, it will yet, if it be at all serious, probably damage his performances on their next several occasions, through his inability to fuse the experiences into a harmonious whole, or, in other words, to understand the ideas which are prescribed to him; for to fuse is only to understand.
And this is absolutely what we find in fact. Mr. Darwin writes concerning hybrids and first crosses:—“The male element may reach the female element, but be incapable of causing an embryo to be developed, as seems to have been the case with some of Thuret’s experiments on Fuci. No explanation can be given of these facts any more than why certain trees cannot be grafted on others.”
I submit that what I have written above supplies a very fair primâ facie explanation.
Mr. Darwin continues:—
“Lastly, an embryo may be developed, and then perish at an early period. This latter alternative has not been sufficiently attended to; but I believe, from observations communicated to me by Mr. Hewitt, who has had great experience in hybridising pheasants and fowls, that the early death of the embryo is a very frequent cause of sterility in first crosses. Mr. Salter has recently given the results of an examination of about five hundred eggs produced from various crosses between three species of Gallus and their hybrids; the majority of these eggs had been fertilised; and in the majority of the fertilised eggs, the embryos had either been partially developed, and had then perished, or had become nearly mature, but the young chickens had been unable to break through the shell. Of the chickens which were born more than four-fifths died within the first few days, or at latest weeks, ‘without any obvious cause, apparently from mere inability to live,’ so that from the five hundred eggs only twelve chickens were reared” (“Origin of Species,” 249, ed. 1876).
No wonder the poor creatures died, distracted as they were by the internal tumult of conflicting memories. But they must have suffered greatly; and the Society for the Prevention of Cruelty to Animals may perhaps think it worth while to keep an eye even on the embryos of hybrids and first crosses. Five hundred creatures puzzled to death is not a pleasant subject for contemplation. Ten or a dozen should, I think, be sufficient for the future.
As regards plants, we read:—
“Hybridised embryos probably often perish in like manner . . . of which fact Max Wichura has given some striking cases with hybrid willows . . . It may be here worth noticing, that in some cases of parthenogenesis, the embryos within the eggs of silk moths, which have not been fertilised, pass through their early stages of development, and then perish like the embryos produced by a cross between distinct species” (Ibid).