Origin of Species Variorum

Comparison with 1866

resemble the mature parent-form. We have seen that this is the rule of development in certain whole groups of animals, as with cuttle-fish, land-shells, fresh-water crustaceans, spiders, and with a few members of the great class of insects. .. .. .. With respect to the final cause of the young in these cases not undergoing any metamorphosis, ... we can see that this would result from the two following contin- gencies: firstly, from the young, during a course of modification carried on for many generations, having to provide at a very early stage of development for their own wants, and secondly, from their following (and this might often be of advantage to a species) exactly the same habits of life with their parents; for in this case, it would be indispensable for the existence of the species, that the child should be modified at a very early age in the same manner with its parents, in accordance with their similar habits. Again, with respect to the singular fact of so many terrestrial and fresh-water animals not undergoing any metamorphosis, whilst the marine members of the same classes pass through various transformations, Fritz Müller has suggested that if an animal during a long succession of generations had to change its habits from living in the sea to living on the land or in fresh-water, it would be a great advantage to its descendants during their modification if they were to lose their metamorphoses; for it is not probable that places well adapted for both the larval and mature stages, under such new and greatly changed habits of life, could be found unoccupied or ill-occupied by other organisms. Therefore the modification of a marine animal into a terrestrial or fresh-water one would generally be much more easily effected, if its metamorphoses were suppressed through the gradual acquirement at an earlier and earlier age of the adult structure. ↑

If, on the other hand, it profited the young to follow habits of life in any slight degree different from those of their parents and consequently to be constructed in a slightly different manner, or if it profited larvæ already having different habits from their parents to change still further their habits, then, on the principle of inheritance at corresponding ages, the young or the larvæ might be rendered by natural selection more and more different from their parents to any conceivable extent. Differences in the larvæ might, also, become correlated with successive stages of development; so that the larvæ in the first stage, might come to differ greatly from the larvæ in the second stage, as is the case with so many animals. The adult might also become fitted for sites or habits, in which the organs of locomotion or of the senses, &c., would be useless; and in this case the final metamorphosis would be said to 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. Fritz Müller, who has recently discussed this whole subject with much ability, goes so far as to believe that the progenitor of all insects probably resembled an adult insect, and that the caterpillar or maggot, and cocoon or pupal stages, have subsequently been acquired; but from this view many naturalists, for instance Sir J. Lubbock, who has likewise recently discussed this subject, would, it is probable, dissent. That certain unusual stages in the metamorphoses of insects have arisen from adaptations to peculiar habits of life can hardly be doubted: thus the first larval form of a certain beetle, the Sitaris, as described by M. Fabre, is a minute, active insect, furnished with six legs, two long antennæ, and four eyes. These larvæ are hatched in the nest of a bee; and when the male-bees emerge in the spring from their burrows, which they do before the females, the larvæ spring on them, and afterwards take an early and natural opportunity of crawling on to the female-bees. When the latter lay their eggs, one in each cell, on the surface of the contained honey, the larva leaps on the egg and devours it. It then undergoes a complete change; its eyes disappear; its legs and antennæ become rudimentary, and it feeds on honey; so that it now more closely resembles the ordinary larvæ of insects; ultimately it undergoes further transformations, and finally emerges as a 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

resemble the mature parent-form. We have seen that this is the rule of development in certain whole groups of animals, as with cuttle-fish and spiders, and with a few members of the great class of insects, as with Aphis. With respect to the final cause of the young in these cases not undergoing any metamorphosis, or closely resembling their parents from their earliest age, we can see that this would result from the two following contingencies; firstly, from the young, during a course of modification carried on for many generations, having to provide for their own wants at a very early stage of development, and secondly, from their following exactly the same habits of life with their parents; for in this case, it would be indispensable for the existence of the species, that the child should be modified at a very early age in the same manner with its parents, in accordance with their similar habits. ↑ Some further explanation, however, of the embryo not undergoing any metamorphosis is perhaps requisite. If, on the other hand, it profited the young to follow habits of life in any degree different from those of their parent, and consequently to be constructed in a slightly different manner, then, on the principle of inheritance at corresponding ages, the active young or larvæ might easily be rendered by natural selection different to any conceivable extent from their parents. Such differences might, also, become correlated with successive stages of development; so that the larvæ, in the first stage, might differ greatly from the larvæ in the second stage, as we have seen to be the case with cirripedes. The adult might become fitted for sites or habits, in which organs of locomotion or of the senses, &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