In presence of this case (and others, though less striking, might be named) there is but one alternative; either we must declare that rabbits and hares form one and the same species—which is absurd—or we must admit that new types may be formed by the union of two existing types; and consequently that species are variable. If the doctrine of Fixity of Species acknowledges the touchstone of hybridity, the fate of the doctrine is settled for ever.
Although I conceive the doctrine of Fixity of Species to be altogether wrong, I cannot say that the arguments adduced in favour of the development hypothesis rise higher than a high degree of probability, still very far from demonstration; they will leave even the most willing disciple beset with difficulties and doubts. When stated in general terms, that hypothesis has a fascinating symmetry and simplicity, but no sooner do we apply it to particular cases, than a thick veil of mystery descends, and our pathway becomes a mere blind groping towards the light. There is nothing but what is perfectly conceivable, and in harmony with all analogies, in the idea of all animal forms having arisen from successive modifications of one original form; but there are many things perfectly conceivable, which have nevertheless no existence; there are many explanations perfectly probable, which are not true; and when we come to seek for the evidence of the development hypothesis, that evidence fails us. It may be true, but we cannot say that it is true. Ten years ago, I espoused the hypothesis, and believed that it must be the truth; but ten years of study, instead of deepening, have loosened that conviction: they have strengthened my opposition to the hypothesis of fixity of species, but they have given greater force to the difficulties which beset the development hypothesis, and have made me feel that at present the requisite evidence is wanting. I conclude with reminding the reader that the question of the origin of species is at present incapable of a positive answer; of the two hypotheses, that of development seems the more harmonious with our knowledge; but it is no more than an hypothesis, and will probably for ever remain one. Now, an hypothesis, although indispensable as a provisional mode of grouping together facts, and giving them some sort of explanation, is after all only a guess, and it may be absurdly wide of the truth. In Natural History, as in all other departments of speculative ingenuity, there have been a goodly number of outrageously extravagant hypotheses, gravely propounded, and credulously accepted. Men prefer an absurd guess to a blank; they would rather have a false opinion than no opinion; and one of the last developments of philosophic culture, is the power of abstaining from forming an opinion, where the necessary data are absent.
If you wish to see how easily hypotheses are formed and accepted, you need only turn over the history of any science. If you want a laugh at credulity, read a chapter of Pliny’s Natural History. Pliny is a classic, and was for centuries an authority; but looked at with impartial eyes, he appears the veriest “old woman” that ever wrote in a beautiful style. He was a mere bookworm, without a particle of scientific insight. His was not an age when men had much regard to evidence; but to him the suspicion never seems to have occurred that Gossip Report could be given to romancing, or that travellers could “see strange things.” No fable is too monstrous for his credulity.
One of the pretty fables Pliny repeats, is, that pearls are formed by drops of dew falling into the gaping valves of the oyster. It never occurred to him to ask whether oysters were ever exposed to the dew? whether the drops could fall into their valves? whether oysters kept their valves open, except when under water? or, finally, whether, if the dew did fall in, it would remain a rounded drop? The drop of dew had a certain superficial resemblance to the pearl, and that was enough. Ælian’s hypothesis was somewhat better: he supposed that the pearls were produced by lightning flashing into the open shells.
Turning from these ancient sages, you will ask how pearls are formed? And almost any ingenious modern, not a zoologist, will tell you (and tell you falsely), that the pearl is a disease of the oyster. One is somewhat fatigued with the merciless frequency with which this notion has been dragged in, as an illustration of genius issuing out of sorrow and adversity; and it is time to stop that “damnable iteration” by discrediting the notion. Know then, that if
“Most wretched men
Are cradled into poetry by wrong:
They learn in suffering what they teach in song”—
it is not true that oysters secrete in suffering what women wear as necklaces. Disease would be the very worst cradle for pearls. The idea of disease originated in a fanciful supposition of pearls being to the oyster and mussel what gall-stones and urinary calculi are to higher and more suffering animals. Réaumur, to whom we owe so many good observations and suggestive ideas, came near the truth when, in 1717, he showed that the structure of pearls was identical with the structure of the shells in which they grow. He attributed their formation to the morbid effusion of coagulating shell-material.
I presume you know that shells are formed by a secretion from the mantle? The mantle is that delicate semi-transparent membrane which you observe, on opening a mussel, lining the whole interior of the shells, and having at its free margins a sort of fringe of delicate tentacles, which are sensitive and retractile. A microscopic examination of these fringes shows them to be glandular in structure—that is, they are secreting organs. The whole mantle, indeed, is a secreting organ, and its secretion is the shell-material: the fringes secrete the colouring matters of the shell, and enlarge its circumference; the rest of the mantle secretes the nacre, or mother-of-pearl, and increases the thickness of the shell. Now it is obvious that the formation of pearl nacre, and of pearls, depends on the healthy condition of the mantle, not on its diseases. If the mantle be injured the nacre is not secreted at all, or in less quantities.