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After what has gone before, the transformation of the Amphibians needs no elucidation. Their predecessors were water-breathers, whose form and mode of life are more faithfully preserved by the long-tailed Amphibians, the tritons, and salamanders, than by the frogs. In our tritons, sexual maturity not rarely commences in the larval state, hence in a phase which was definitive in the progenitors of the present genera. There is, indeed, one species, the Mexican Axolotl, which normally propagates itself during the larval phase. Auguste
Dumeril's observation is highly interesting, that of the thousands of Axolotls that he bred at Paris, some few advanced beyond the grade of development hitherto known in them, i.e. they lost their gills, changed the
shape of their bodies not inconsiderably, and from gillbreathers, and aquatic animals became lung-breathers and terrestrial animals. It needs further observation to ascertain whether (what is, however, very improbable), in their home, all Axolotls, after having propagated themselves in their larval state, undergo the metamorphosis into salamander-like animals (Amblystoma), or whether the transfer to Europe and the consequent entire change of the circumstances of life gave the impulse to a progressive transformation of these few individuals, which, by the continuance of these conditions, would in future generations extend to more and more individuals, and finally become the characteristic of a new species.
The examples of Ontogenesis, or individual development, hitherto examined, had the peculiarity that the sexual animal does not issue directly from its egg like the Phoenix from its ashes, but had to pass through various forms and existences in which the progenitors of the species again become alive and palpable. We must now inquire how this development is related to that form of reproduction which the systematizers, completely in accordance with the facts, yet without any corresponding meaning, have termed "direct development," or "development without heterogenesis or metamorphosis?" The ciliated embryos of many Medusæ are not converted into polype-like intermediate forms, but pass directly into Medusa. The greater number of higher crabs do not leave the egg as Nauplia, but as more or less perfect decapods. The bird, the mammal, and man are all at birth "similar to their parents." Con- . sidering that the processes of heterogenesis are in themselves by no means advantageous to or "in harmony
with design"-we have only to remember the fate of the tapeworm's eggs-that by the larval state the period of infancy and weakness is prolonged, and the period of maturity and efficient care for the continuance of the species delayed, it follows that curtailments and reductions, consequent on adaptation have, as advantageous modifications, a prospect of perpetuation. As in Amphibians the prolongation of the larval phase may be effected by natural circumstances and artificial experiments, so in like manner a compression of the phases of transformation, and a general curtailment of the metamorphosis is imaginable. In the class of Amphibians we have, in fact, several examples of curtailed and modified metamorphosis which bridge over the apparent chasm between development with and without transformation, and render direct development comprehensible as being gradually acquired. Amphibians will endcavour to extend themselves wherever they are invited by a sufficient supply of insects, and the black salamander of the mountains (Salamandra atra) has even overcome the impediment which might have been deemed insurmountable, the absence of water for its larvæ. It does not lay its eggs like its congeners, but only two are received into the oviduct, and the fluids secreted from its walls replace the marsh to them and to the larvæ which emerge from them. Here, and not when separated from the parent, do the gills make their appearance, while the other eggs, gradually following, are devoured by the hungry larvæ. The metamorphosis of the black salamander, respecting which, unluckily, no recent investigations have been made, thus takes place within the parental body, and there is no difficulty in imagining
the acquisition of this peculiarity by the necessity of adaptation. If the mode of life of the marsupial frog, which carries its young in a membranous fold of the back, and the Surinam toad, of which the larvæ live singly in the chambers of a kind of honeycomb on the back, were better known than they are, we should assuredly arrive at the same results as with the black salamander. In the absence of other knowledge, the observations of M. Bavey, Marine Pharmaceutist at Guadaloupe, first published in 1873, are of the highest importance. A frog of those parts (Hylodon Martinicensis) goes through its whole metamorphosis in the egg. In the egg it has gills and tail; and from the brief remark that the island contains only rapid running streams, and nowhere stagnant waters or marshes, it appears that this is also a case in which adaptation modifies and curtails development.
If, after this introduction, we now examine the socalled direct development with more attention, it may in every way be compared to the metamorphosis of the Hylodes of Guadaloupe. Direct development is a transformation in the ovum; and in the cases in which it occurs, the phases of embryonic development are repetitions, more or less distinct, of the historic development of the family. We will only particularize in the embryonic life of the Vertebrata (in which metamorphosis does not take place), some phases that are stages of curtailed transformation, and recapitulate the permanent condition of their progenitors. It has been repeatedly mentioned that in all vertebrate animals, the vertebral column is first laid out as an unsegmented cord and an unsegmented sheath for the
spinal cord. This is the permanent state of the lower fishes. In the higher Vertebrata also, the brain at first consists of vesicles, lying one behind the other, which is the persistent form of the lower groups. The embryonic heart of mammals and birds begins in the form of a tube, and subsequently acquires the communications between the chambers, which in the reptiles never close. In the Amphibians, the branchial arches really bear gills during the larval state. They are not wanting in the embryos of reptiles, birds, and mammals, any more than the fissures through which, in fish and the larvæ of Amphibians, the water passes off after being inhaled. Must we again set forth the only possible explanation of these facts?
Before referring to the phenomena which testify the emanation of families from a common root, we will cite one of the most important evidences of recent times, which traces the genesis of species through a great geological period, and exhibits in detail the relations. of the development of the individuals to that of the species, genus, and family. We mean L. Würtenberger's contribution to the geological evidence of the Darwinian theory, to which we have already appealed (p. 97). It relates to the two families of Ammonites, the Planulata and Armata; of which, according to Würtenberger's researches, the latter are developed from the former, as the ribs of the Planulata gradually pass into the spines of the Armata. Of special interest to us are the following passages of the preliminary communication on the discoveries obtained from thousands of specimens, . and which will probably not be made public, with all the vouchers, for some years to come. "It gave me parti