It is not a little remarkable that observations so precise as these should have for many years passed unregarded, and not led to the true explanation of the mystery. Perhaps an inherent love of the marvellous made men greedily accept the idea of resuscitation, and indisposed them to attempt an explanation of it. Spallanzani’s own attempt is certainly not felicitous. He supposes that the dust prevents the lacerating influence of the air from irritating and injuring the animals. And this explanation is accepted by his Translator.
[Since the foregoing remarks were in type, M. Gavarret has published (Annales des Sciences Naturelles, 1859, xi. p. 315) the account of his experiments on Rotifers and Tardigrades, in which he found that after subjecting the moss to a desiccation the most complete according to our present means, the animals revived after twenty-four hours’ immersion of the moss in water. This result seems flatly to contradict the result I arrived at; but only seems to contradict it, for in my experiments the animals, not the moss, were subjected to desiccation. Nevertheless, I confess that my confidence was shaken by experiments so precise, and performed by so distinguished an investigator, and I once more resumed the experiments, feeling persuaded that the detection of the fallacy, wherever it might be, would be well worth the trouble. The results of these controlling experiments are all I can find room for here:—Whenever the animals were completely separated from the dirt, they perished; in two cases there was a very little dirt—a mere film, so to speak—in the watch-glass, and glass-cell, and this, slight as it was, sufficed to protect two out of eight, and three out of ten Rotifers, which revived on the second day; the others did not revive even on the third day after their immersion. In one instance, a thin covering-glass was placed over the water on the slide, and the evaporation of the water seemed complete, yet this glass-cover sufficed to protect a Rotifer, which revived in three hours.
If we compare these results with those obtained by M. Davaine, we can scarcely avoid the conclusion that it is only when the desiccation of the Rotifers is prevented by the presence of a small quantity of moss, or of dirt—between the particles of which they find shelter—that they revive on the application of water. And even in the severe experiments of M. Doyère and M. Gavarret, some of the animals must have been thus protected; and I call particular attention to the fact that, although some animals revived, others always perished. But if the organization of the Rotifer, or Tardigrade, is such that it can withstand desiccation—if it only needs the fresh applications of moisture to restore its activity—all, or almost all, the animals experimented on ought to revive; and the fact that only some revive leads us to suspect that these have not been desiccated—a suspicion which is warranted by direct experiments. I believe, then, that the discrepancy amounts to this: investigators who have desiccated the moss containing animals, find some of the animals revive on the application of moisture; but those who desiccate the animals themselves, will find no instances of revival.]
The time spent on these Rotifers will not have been misspent if it has taught us the necessity of caution in all experimental inquiries. Although Experiment is valuable—nay, indispensable—as a means of interrogating Nature, it is constantly liable to mislead us into the idea that we have rightly interrogated, and rightly interpreted the replies; and this danger arises from the complexity of the cases with which we are dealing, and our proneness to overlook, or disregard, some seemingly trifling condition—a trifle which may turn out of the utmost importance. The one reason why the study of Science is valuable as a means of culture, over and above its own immediate objects, is that in it the mind learns to submit to realities, instead of thrusting its figments in the place of realities—endeavours to ascertain accurately what the order of Nature is, and not what it ought to be, or might be. The one reason why, of all sciences, Biology is pre-eminent as a means of culture, is, that owing to the great complexity of all the cases it investigates, it familiarizes the mind with the necessity of attending to all the conditions, and it thus keeps the mind alert. It cultivates caution, which, considering the tendency there is in men to “anticipate Nature,” is a mental tonic of inestimable worth. I am far from asserting that biologists are more accurate reasoners than other men; indeed, the mass of crude hypothesis which passes unchallenged by them, is against such an idea. But whether its advantages be used or neglected, the truth nevertheless is, that Biology, from the complexity of its problems, and the necessity of incessant verification of its details, offers greater advantages for culture than any other branch of science.
I have once or twice mentioned the words Mollusc and Crustacean, to which the reader unfamiliar with the language of Natural History will have attached but vague ideas; and although I wanted to explain these, and convey a distinct conception of the general facts of Classification, it would have then been too great an interruption. So I will here make an opportunity, and finish the chapter with an indication of the five Types, or plans of structure, under one of which every animal is classed. Without being versed in science, you discern at once whether the book before you is mathematical, physical, chemical, botanical, or physiological. In like manner, without being versed in Natural History, you ought to know whether the animal before you belongs to the Vertebrata, Mollusca, Articulata, Radiata, or Protozoa.
Fig. 17.
Male Triton, or Water-Newt.
A glance at the contents of our glass vases will yield us samples of each of these five divisions of the animal kingdom. We begin with this Triton. It is a representative of the Vertebrate division, or sub-kingdom. You have merely to remember that it possesses a backbone and an internal skeleton, and you will at once recognize the cardinal character which makes this Triton range under the same general head as men, elephants, whales, birds, reptiles, or fishes. All these, in spite of their manifold differences, have this one character in common:—they are all backboned; they have all an internal skeleton; they are all formed according to one general type. In all vertebrate animals the skeleton is found to be identical in plan. Every bone in the body of a triton has its corresponding bone in the body of a man, or of a mouse; and every bone preserves the same connection with other bones, no matter how unlike may be the various limbs in which we detect its presence. Thus, widely as the arm of a man differs from the fin of a whale, or the wing of a bird, or the wing of a bat, or the leg of a horse, the same number of bones, and the same connections of the bones, are found in each. A fin is one modified form of the typical limb; an arm is another; a wing another. That which is true of the limbs, is also true of all the organs; and it is on this ground that we speak of the vertebrate type. From fish to man one common plan of structure prevails; and the presence of a backbone is the index by which to recognize this plan.