Facts and Arguments for Darwin by Fritz Muller (read along books .TXT) 📕

But let us stick to the Crustacea. In Polyphemus Leydig finds the first traces of the intestinal tube even during segmentation. In Mysis a provisional tail is first formed, and in Ligia a maggot-like larva-skin. The simple median eye appears earlier, and would therefore be more important than the compound paired eyes; the scale of the antennae in the Prawns would be more important than the flagellum; the maxillipedes of the Decapoda would be more important than the chelae and ambulatory feet, and the anterior six pairs of feet in the Isopoda, than the precisely similarly formed seventh pair; in the Amphipoda the most important of all organs would be the “micropylar apparatus,” which disappears without leaving a trace soon after hatching; in Cyclops the setae of the tail would be more important than all the natatory feet; in the Cirripedia the posterior antennae, as to which we do not know what becomes of them, would be more important than the cirri, and so forth. The most unimportant of all organs would be the sexual organs, and the most essential peculiarity would consist in colour, which is to be referred back to the ovarian egg.

“The embryos, or young states of different animals, resemble each other the more, the younger they are,” or, as Johannes Muller expresses it, “they approach the more closely to the common type.” Different as may be the ideas connected with the word “type,” no one will dispute that the typical form of the penultimate pair of feet in the Amphipoda is that of a simple ambulatory foot, and not that of a chela, for the latter occurs in no single adult Amphipod; we know it only in the young of the genus Brachyscelus, which therefore in this respect undoubtedly depart more widely than the adults from the type of their order. This applies also to the young males of the Shore-hoppers (Orchestia) with regard to the second pair of anterior feet (gnathopoda). In like manner no one will hesitate to accept the possession of seven pairs of feet as a “typical” peculiarity of the Edriophthalma, which Agassiz, on this account, names Tetradecapoda; the young Isopoda, which are Dodecapoda, are also in this respect further from the “type” than the adults.

It is certainly a rule, and this Darwin’s theory would lead us to expect, that in the progress of development those forms which are at first similar gradually depart further from each other; but here, as in other classes, the exceptions, for which the Old School has no explanation, are numerous. Not unfrequently we might indeed directly reverse the proposition and assert that the difference becomes the greater, the further we go back in the development, and this not only in those cases in which one of two nearly allied species is directly developed, and the other passes through several larval stages, such as the common Crayfish and the Prawns which are produced from Nauplius-brood. The same may be said, for example, of the Isopoda and Amphipoda. In the adult animals the number of limbs is the same; at the first sight of a Cyrtophium or a Dulichia, and even after the careful examination of a Tanais, we may be in doubt whether we have an Isopod or an Amphipod before us; in the newly-hatched young the number of limbs is different, and if we go back to their existence in the egg, the most passing glance to see whether the curvature is upwards or downwards suffices to distinguish even the youngest embryos of the two orders.

In other instances, the courses which lead from a similar starting-point to a similar goal, separate widely in the middle of the development, as in the Prawns with Nauplius-brood already described.

Finally, so that even the last possibility may be exhausted, it sometimes happens that the greatest similarity occurs in the middle of the development. The most striking example of this is furnished by the Cirripedia and Rhizocephala, whether we compare the two orders or the members of each with one another; from a segmentation quite different in its course (see Figures 61 to 64) proceed different forms of Nauplius, these become converted into exceedingly similar pupae, and from the pupae again proceed sexually mature animals, differing from each other toto coelo.

“If the formation of the organs occurs in the order corresponding to their importance, this sequence must of itself be a criterion of their comparative value in classification.” THAT IS TO SAY, SUPPOSING THE PHYSIOLOGICAL AND CLASSIFICATIONAL VALUE OF AN ORGAN TO COINCIDE! Just as in Christian countries there is a catechismal morality, which every one has upon his lips, but no one considers himself bound to follow, or expects to see followed by anybody else, so also has Zoology its dogmas, which are as universally acknowledged, as they are disregarded in practice. Such a dogma as this is the supposition tacitly made by Agassiz. Of a hundred who feel themselves compelled to give their systematic confession of faith as the introduction to a Manual or Monographic Memoir, ninety-nine will commence by saying that a natural system cannot be founded upon a single character, but that it has to take into account all characters, and the general structure of the animal, but that we must not simply sum up these characters like equivalent magnitudes, that we must not count but weigh them, and determine the importance to be ascribed to each of them according to its physiological significance. This is probably followed by a little jingle of words in general terms on the comparative importance of animal and vegetative organs, circulation, respiration, and the like. But when we come to the work itself, to the discrimination and arrangement of the species, genera, families, etc., in all probability not one of the ninety-nine will pay the least attention to these fine rules, or undertake the hopeless attempt to carry them out in detail. Agassiz, for example, like Cuvier, and in opposition to the majority of the German and English zoologists, regards the Radiata as one of the great primary divisions of the Animal Kingdom, although no one knows anything about the significance of the radiate arrangement in the life of these animals, and notwithstanding that the radiate Echinodermata are produced from bilateral larvae. The “true Fishes” are divided by him into Ctenoids and Cycloids, according as the posterior margin of their scales is denticulated or smooth, a circumstance the importance of which to the animal must be infinitely small, in comparison to the peculiarities of the dentition, formation of the fins, number of vertebrae, etc.

And, to return to our Class of the Crustacea, has any particular attention been paid in their classification to the distinctions prevailing in the “most essential organs”? For instance, to the nervous system? In the Corycaeidae, Claus found all the ventral ganglia fused together into a single broad mass, and in the Calanidae a long ventral chain of ganglia,—the former, therefore, in this respect resembling the Spider Crabs and the latter the Lobster; but no one would dream on this account of supposing that there was a relationship between the Corycaeidae and the Crabs, or the Calanidae and the Lobsters.—Or to the organs of circulation? We have among the Copepoda, the Cyclopidae and Corycaeidae without a heart, side by side with the Calanidae and Pontellidae with a heart. And in the same way among the Ostracoda, the Cypridinae, which I find possess a heart, place themselves side by side with Cypris and Cythere which have no such organ.—Or to the respiratory apparatus? Milne-Edwards did this when he separated Mysis and Leucifer from the Decapoda, but he himself afterwards saw that this was an error. In one Cypridina I find branchiae of considerable size, which are entirely wanting in another species, but this does not appear to me to be a reason for separating these species even generically.

On the other hand, what do we know of the physiological significance of the number of segments, and all the other matters which we are accustomed to regard as typical peculiarities of the different organs, and to which we usually ascribe the highest systematic value?

“Those peculiarities which first appear, should be more highly estimated than those which appear subsequently. A system, in order to be true and natural, must agree with the sequence of the organs in the development of the embryo.” If the earlier manifested peculiarities are to be estimated more highly than those which afterwards make their appearance, then in those cases in which the structure of the adult animal requires one position in the system, and that of the larva another, the latter and not the former must decide the point. As the Lernaeae and Cirripedes, on account of their Nauplius-brood, were separated from their previous connexions and referred to the Crustacea, we shall, for the same reason, have to separate Peneus from the Prawns and unite it with the Copepoda and Cirripedia. But the most zealous embryomaniac would probably shrink from this course.

A “true and natural system” of the Crustacea to be in accordance with the sequence of the phenomena would have to take into account in the first place the various modes of segmentation, then the position of the embryo, next the number of limbs produced within the egg and so forth, and might be represented somewhat as follows:—

CLASSIS CRUSTACEA.

Subclass I. HOLOSCHISTA.—Segmentation complete. No primitive band. Nauplius-brood.

Ord. 1. Ceratometopa.—Nauplius with frontal horns. (Cirripedia, Rhizocephala.)

Ord. 2. LEIOMETOPA.—Nauplius without frontal horns. (Copepoda, without Achtheus, etc., Phyllopoda, Peneus.)

Subclass II. HEMISCHISTA.—Segmentation not complete.

A. Nototropa.—Embryo bent upwards.

Ord. 3. Protura.—The tail is first formed. (Mysis.)

Ord. 3. Saccomorpha.—A maggot-like larva-skin is first formed. (Isopoda.)

B. Gasterotropa.—Embryo bent ventrally.

Ord. 5. Zoeogona.—Full number of limbs not produced in the egg. Zoea-brood. (The majority of the Podophthalmata.)

Ord. 6. Ametabola.—Full number of limbs produced in the egg. (Astacus, Gecarcinus, Amphipoda less Hyperia ?)

This sample may suffice. The farther we go into details in this direction, the more brilliantly, as may easily be imagined, does the naturalness of such an arrangement as this force itself upon us.

All things considered, we may apply the judgment which Agassiz pronounced upon Darwin’s theory, with far greater justice to the propositions just examined:—“No theory,” says he, “however plausible it may be, can be admitted in science, unless it is supported by facts.”

CHAPTER 11. ON THE PROGRESS OF EVOLUTION.

From this scarcely unavoidable but unsatisfactory side-glance upon the old school, which looks down with so great an air of superiority upon Darwin’s “intellectual dream” and the “giddy enthusiasm” of its friends, I turn to the more congenial task of considering the developmental history of the Crustacea from the point of view of the Darwinian theory.

Darwin himself, in the thirteenth chapter of his book, has already discussed the conclusions derived from his hypotheses in the domain of developmental history. For a more detailed application of them, however, it is necessary in the first place to trace these general conclusions a little further than he has there done.

The changes by which young animals depart from their parents, and the gradual accumulation of which causes the production of new species, genera, and families, may occur at an earlier or later period of life,—in the young state, or at the period of sexual maturity. For the latter is by no means always, as in the Insecta, a period of repose; most other animals even then continue to grow and to undergo changes. (See above, the remarks on the