duality; and, moreover, a similar diversity is observed in the specific differences among them; or, in other words, a greater or less degree of polymorphism is remarked. With Ctenophora (or Ciliogrades of De Blainville) this polymorphic tendency is at a minimum; for here, not only are the individuals composing the group closely similar, but being all hermaphrodite, there is not even the polymorphism arising from difference of sex. This, however, does occur in the Pulmograde Discophora (to which our naked-eyed Medusæ belong), and sometimes the variations are very striking, as in Aurelia, one of the covered-eyed division, which has received from writers the names, Aurelia lineolata (Peron), A. radiolata (Lamarck) A. granulata, A. rosea, A. surirea, A. purpurata, Medusa purpurata (Penn), and Biblis Aquitaniæ (Lesson), the species indicated being in every case Aurelia aurita (Forbes). Deviations from the normal number of parts constitute another source of polymorphism. Next, the cycle of individual differences embraces two distinct types of individuals—the Medusa type and the Hydra type. One of these types may exhibit more or less diversity, there being frequently two kinds of Hydra united in one and the same community; or (though more rarely), two kinds of Medusæ, as among the Siphonophora (Physogrades). Thus, in the Diphyde, which appear like pieces of transparent glass, and which were imagined by Cuvier and others to consist each of two distinct animals, always united, although separable with impunity, Professor Huxley shewed (Phil. Trans., 1849) that they consist of two constantly associated, though easily separated forms, slightly attached, but capable, for some time at least, of an independent existence. These two Medusa forms; one (anterior natatory body) including the other (posterior natatory body), may be very similar, as in Diphyes, or very dissimilar, as in Cuboides vitreus. In the latter, the including (anterior) individual is large and cuboid, the included (posterior) individual is small, tetragonal, and campanulate; whereas in Abyla trigona, the reverse occurs, the including individual being here small, subcuboid, and campanulate; and the included, much larger, oblong, and polygonal.* Agassiz goes on to argue from all this, as follows--"But notwithstanding the polymorphism among the individuals of one and the same community, genetically connected together, each successive generation reproduces the same kinds of heterogeneous individuals, and nothing but individuals linked together in the same way. Surely we have here a much greater diversity of individuals, born one from the other, than is exhibited by the most diversified breeds of our domesticated animals; and yet all these heterogeneous individuals remain true to their species, in one case as in the other, and do not afford the slightest evidence of a transmutation of species." It is immediately after this that the passage follows, the objection to which has given rise to these remarks—“Would," says Agassiz, "the supporters of the fanciful theories, lately propounded, only extend their studies a little beyond the range of domesticated animals-would they investigate the alternate generations of the Acalephs-the extraordinary modes of development of the Helminth-the reproduction of the Salpæ, &c.,-they would soon learn that there are in the world far more astonishing phenomena, strictly circumscribed between the natural limits of unvarying species, than the slight differences produced by the intervention of man among domesticated animals, and, perhaps, cease to be so confident * In Prof. Huxley's elaborate Monograph of the Oceanic Hydrozoa (Ray Society, 1859) the Cirrhigrada and Physograda of De Blainville are recast and differently arranged, forming the two families, Calycophorida and Physophoridæ. The including and included individuals spoken of in the text are regarded by him as organs of propulsion, and distinguished as proximal and distal nectocalyx. I have retained Cuboides as an illustration, because it does not appear that this is interfered with by Huxley's supposition that it is a Diphyozoöid, derived from Abyla. In Hippopodius, a genus of the Calycophoridæ, the nectocalyces are said to be as many as twelve in number. as they seem to be, that these differences are trustworthy indications of the variability of species." I have here fairly stated Agassiz' views-in fact quoted them nearly verbatim, simply adding illustrations; and, I need scarcely remark, in the first place, that these "astonishing phenomena❞ can, in no respect, be imagined to be novelties to M. Agassiz, who, thirteen years ago, published, in conjunction with A. A. Gould, the admirable "Outlines of Comparative Physiology," in which a chapter is devoted to a lucid exposition of these very changes. In the second place, that in Darwin's work on the "Origin of Species," the subject of the "alternate generations of Acalephs, the extraordinary modes of development of the Helminth, and the reproduction of the Salpæ,” are altogether ignored, and find no place in the argument, being nowhere, in the remotest manner, alluded to. From this I deduce three things:-First, that no man had more cause than Agassiz, by reason of his thorough acquaintance with the subject in all its bearings, to be impressed with the vast importance of these polymorphisms and alternations in any question of biology so comprehensive as the Origin of Species. Secondly, I think we have every right to infer that these most curious and astonishing phenomena (notwithstanding the elaborate notice which he takes of the somewhat analogous phenomena of neuter insects) had not been regarded by Darwin in so important a light as to make him consider them a necessary part of his argument, or a possible objection to his theory; and, thirdly, that Agassiz, whose opinion on the question must command the highest respect, by calling attention to the omission, has done nothing more nor less than might reasonably have been looked for from so distinguished a Physiologist. But I still maintain that the object of the whole reasoning of Agassiz in the passage quoted is simply to draw the reader's attention to the fact of the great extent to which polymorphism obtains among Acalephs, and that he nowhere states that he regards the metamorphoses undergone by them in the alternations of generations, in the light of varieties. He simply includes, and very properly, the cycle of ovum, hydroid, and medusoid, in one polymorphic species; and the illustration is, therefore, perfectly just. But if Agassiz had definitely stated his opinion that the transformations of the Medusæ could be regarded as varieties, his doctrine would not have been so heretical but that he would have received the support of many eminent physiologists, and among them of the late illustrious Professor E. Forbes, than whom few had more closely studied the Acalephæ, as his beautful Monograph, published by the Ray Society, amply testifies. At page 82 of that work, he says-"In what light are we to regard the relationship between the Medusa and Polyp? The one is not the larva of the other, as is often improperly said, because there is no metamorphosis of the one into the other. The first is the parent of the last, and the last of the first, but neither is a stage of an individual existence, destined to begin life as a Medusa and end it as a Poloyp, and vice versa. In the case of Aurelia a. The Medusa produces eggs; b. The eggs produce Infusoria; c. The Infusoria fix, and become hydroid Polyps ; With such facts, unquestioned facts, before us, it seems to me that we have no choice between theories, and we must admit the idea of alternation of generations to be true.” In point of fact, however, the phenomena of alternate generations, or "the production of dissimilar individuals among sexual animals, by a non-sexual process" (Allen Thomson) are far more astonishing than the transformations undergone by insects. For there is only the most superficial analogy between the alternation of generations of Medusa and Salpæ, and the metamorphoses of insects. In the latter there is a distinct change from one stage into another, readily traceable; in the former, the animals "remain different through their whole life, so that their relationship does not appear until a succeeding generation. The son does not resemble the father, but the grandfather; and in some cases the resemblance reappears only at the fourth or fifth generation (as in Distoma) or even later" (as in Aphis, at the ninth.) Thus in the case of the Acalephs-the little animal, which on leaving the egg, has the form of an infusory, passes in succession through the phases of Scyphistoma, Strobila and Ephyræ, so called, because, before these changes were understood, they were imagined to be different genera, and were named accordingly. "But the remarkable point in these metamorphoses is, that what was at first a single individual, is thus transformed by transverse division into a number of entirely distinct animals, which is not the case in ordinary metamorphoses. Moreover the upper segment [of the strobila] does not follow the others in their development. Its office seems to be accomplished as soon as the other segments begin to be independent; being intended merely to favour their development, by securing and preparing the substances necessary to their growth." Hence they are called Medusa polypiform nurses. "There is [then] this essential difference between the metamorphoses of the caterpillar, and alternate reproduction, that, in the former case, the same individual passes through all the phases of development; whereas, in the latter, the individual disappears, and makes way for another, which carries out what its predecessors had begun. It would give a correct idea of this difference to suppose that the tadpole, instead of being itself transformed into a frog, should die, having first brought forth young frogs; or that the chrysalis should, in the same way, produce young butterflies. In either case the young would still belong H |