Facts and Arguments for Darwin

by Fritz Müller

In 1856, in an elementary work,[^[3]] in which it is usual to admit only what are regarded as the assured acquisitions of science, Agassiz expresses himself as follows:—

“The ovarian eggs of all animals are perfectly identical, small cells with a vitellus, germinal vesicle and germinal spot” (§ 278). “The organs of the body are formed in the sequence of their organic importance; the most essential always appear first. Thus the organs of vegetative life, the intestine, etc., appear later than those of animal life, the nervous system, skeleton, etc., and these in turn are preceded by the more general phenomena belonging to the animal as such” (§ 318). “Thus, in Fishes, the first changes consist in the segmentation of the vitellus and the formation of a germ, processes which are common to all classes of animals. Then the dorsal furrow, characteristic of the Vertebrate, appears—the brain, the organs of the senses; at a later period are formed the intestine, the limbs, and the permanent form of the respiratory organs, from which the class is recognised with certainty. It is only after exclusion that the peculiarities of the structure of the teeth and fins indicate the genus and species” (§ 319). “Hence the embryos of different animals resemble each other the more, the younger they are” (§ 320). “Consequently the high importance of developmental history is indubitable. For, if the formation of the organs takes place in the order corresponding to their importance, this sequence must of itself be a criterion of their comparative value in classification. The peculiarities which appear earlier should be considered of higher value than those which appear subsequently” (§ 321). “A system, in order to be true and natural, must agree with the sequence of the organs in the development of the embryo” (§ 322).

I do not know whether any one at the present day will be inclined to subscribe to this proposition in its whole extent.[^[4]] It is certain, however, that views essentially similar are still to be met with everywhere in discussions on classification, and that even within the last few years, the very sparingly successful attempts to employ developmental history as the foundation of classification have been repeated.

But how do these propositions agree with our observations on the developmental history of the Crustacea? That these observations relate for the most part to their “free metamorphosis” after their quitting the egg, cannot prejudice their application to the propositions enunciated especially with regard to “embryonal development” in the egg; for Agassiz himself points out (§ 391) that both kinds of change are of the same nature and of equal importance and that no “radical distinction” is produced by the circumstance that the former take place before and the latter after birth.

“The ovarian eggs of all animals are identical, small cells with vitellus, germinal vesicle and germinal spot.” Yes, somewhat as all Insects are identical, small animals with head, thorax, and abdomen; that is to say if, only noticing what is common to them, we leave out of consideration the difference of their development, the presence or absence and the multifarious structure of the vitelline membrane, the varying composition of the vitellus, the different number and formation of the germinal spots, etc. Numerous examples, which might easily be augmented, of such profound differences, are furnished by Leydig’s ‘Lehrbuch der Histologie.’ In the Crustacea the ovarian eggs actually sometimes furnish excellent characters for the discrimination of species of the same genus; thus, for example, in one Porcellana of this country they are blackish-green, in a second deep blood-red, and in a third dark yellow; and within the limits of the same order they present considerable differences in size, which, as Van Beneden and Claus have already pointed out, stands in intimate connexion with the subsequent mode of development.

“The organs of the body are formed in the sequence of their organic importance; the most essential always appear first.” This proposition might be characterised à priori as undemonstrable, since it is impossible either in general, or for any particular animal, to establish a sequence of importance amongst equally indispensable parts. Which is the more important, the lung or the heart—the liver or the kidney?—the artery or the vein? Instead of giving the preference, with Agassiz, to the organs of animal life, we might with equal justice give it to those of vegetative life, as the latter are conceivable without the former, but not the former without the latter. We might urge that, according to this proposition, provisional organs as the first produced must exceed the later-formed permanent organs in importance.

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 antennæ in the Prawns would be more important than the flagellum; the maxillipedes of the Decapoda would be more important than the chelæ 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 setæ of the tail would be more important than all the natatory feet; in the Cirripedia the posterior antennæ, 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 Müller 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.