Origin of Species Variorum

Comparison with 1872

&c., would be useless; and in this case the .. metamorphosis would be ... retrograde. ↑

From the remarks above made we can see how by alterations of structure in the young, in conformity with altered habits of life, together with inheritance at corresponding ages, the metamorphoses of certain animals might first have been acquired, and subsequently transmitted to numerous modified descendants. Most of our best authorities are now convinced that the various larval and pupal stages of insects have thus been acquired through adaptation, and not through inheritance from some ancient form. The curious case of Sitaris— a beetle which passes through certain unusual stages of development— will illustrate how this might occur. The first larval form is described by M. Fabre, as an active, minute insect, furnished with six legs, two long antennæ, and four eyes. These larvæ are hatched in the nests of bees; and when the male-bees emerge from their burrows, in the spring, which they do before the females, the larvæ spring on them, and afterwards crawl on to the females whilst paired with the males. As soon as the female bee deposits her eggs on the surface of the honey stored in the cells, the larvæ of the Sitaris leap on the eggs and devour them. Afterwards they undergo a complete change; their eyes disappear; their legs and antennæ become rudimentary, and they feed on honey; so that they now more closely resemble the ordinary larvæ of insects; ultimately they undergo a further transformation, and finally emerge as the perfect beetle. ↑ Now, if an insect, undergoing transformations like those of the Sitaris, had been the progenitor of the whole great class of insects, the general course of development, and especially that of the first larval stage, would probably have been widely different from what is actually the case; and it should be especially noted that the first larval stage would not have represented the adult condition of any insect.

On the other hand it is probable that with many groups of animals the earlier larval stages do show us, more or less completely, the form of the ancient and adult progenitor of the whole group. In the enormous class of the Crustacea, forms wonderfully distinct from each other, as the suctorial parasites, cirripedes, entomostraca, and even the malacostraca, appear in their first larval state under a similar nauplius form; and as these larvæ feed and live in the open sea, and are not adapted for any peculiar habits of life, and from other reasons assigned by Fritz Müller, it is probable that an independent adult animal, resembling the nauplius, formerly existed at a remote period, and has subsequently produced, through long-continued modification along several divergent lines of descent, the several above-named great Crustacean groups. So again it is probable, from what we know of the embryos of mammals, birds, fishes, and reptiles, that all the members in these four great classes are the modified descendants of some one ancient progenitor, which was furnished in its adult state with branchiæ, had a swim-bladder, four simple limbs, and a long tail fitted for an aquatic life.

As all the organic beings, extinct and recent, which have ever lived, can be arranged within a few great classes; and as all within each class have, according to our theory, been connected together by fine gradations, the best, and, if our collections were nearly perfect, the only possible arrangement, would be genealogical. Descent being ... the hidden bond of connexion which naturalists have been seeking under the term of the Natural System. On this view we can understand how it is that, in the eyes of most naturalists, the structure of the embryo is even more important for classification than that of the adult. ↑ In two or more groups of animal, however much they may .. differ from each other in structure and habits in their adult condition, if they pass through closely similar embryonic stages, we may feel assured that they all are descended from one parent-form, and are therefore ... closely related. Thus, community in embryonic structure reveals community of descent; but dissimilarity in embryonic development does not prove discommunity of descent, for in one of two groups the developmental stages may have been suppressed, or may have been so greatly modified through adaptation to new habits of life, as to be no longer recognisable. Even in groups, in which the adults have been modified to an extreme degree, community of origin is often revealed by the structure of the larvæ; we have seen, for instance, that cirripedes, though externally so like shell-fish, are at once known by their larvæ to belong to the great class of crustaceans. ↑ As the embryo often shows us more or less plainly the structure of the less modified and ancient progenitor of the group, we can .. see why ancient and extinct forms so often resemble in their adult state the embryos of ... existing species of the same class. Agassiz believes this to be a universal law of nature; and we may hope hereafter to see the law proved true. It can, however, be proved true only in those cases .. in which the ancient state

&c., would be useless; and in this case the final metamorphosis would be said to be retrograde. ↑ ↑

As all the organic beings, extinct and recent, which have ever lived on this earth have to be classed together, and as all have been connected by fine gradations, the best, or indeed, if our collections were nearly perfect, the only possible arrangement, would be genealogical; descent being on my view the hidden bond of connexion which naturalists have been seeking under the term of the natural system. On this view we can understand how it is that, in the eyes of most naturalists, the structure of the embryo is even more important for classification than that of the adult. For the embryo is the animal in its less modified state; and in so far it reveals the structure of its progenitor. In two groups of animals, however much they may at present differ from each other in structure and habits, if they pass through the same or similar embryonic stages, we may feel assured that they have both descended from the same or nearly similar parents, and are therefore in that degree closely related. Thus, community in embryonic structure reveals community of descent. It will reveal this community of descent, however much the structure of the adult may have been modified and obscured; we have seen, for instance, that cirripedes can at once be recognised by their larvæ as belonging to the great class of crustaceans. As the embryonic state of each species and group of species partially shows us the structure of their less modified ancient progenitors, we can clearly see why ancient and extinct forms of life should resemble the embryos of their descendants,— our existing species. Agassiz believes this to be a law of nature; but I am bound to confess that I only hope to see the law hereafter proved true. It can be proved true in those cases alone in which the ancient state,