Origin of Species Variorum

Comparison with 1869

at a very early period of life, or the steps may have been inherited at an earlier age than that at which they first occurred. In either case, as we have seen with the short-faced tumbler, the young or embryo would closely resemble the mature parent-form. And this is the rule of development in certain whole groups or sub-groups, as with cuttle-fish, land-shells, fresh-water crustaceans, spiders, and some members of the great class of insects. .. .. .. With respect to the final cause of the young in these groups not passing through any metamorphosis, ... we can see that this would follow from the .. following contingencies; namely, from the young ... having to provide at a very early age for their own wants, and from their following ... the same habits of life with their parents; for in this case, it would be indispensable for their existence that they ... should be modified ... in the same manner as their parents. ↑ 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. In this case the gradual acquirement at an earlier and earlier age of the adult structure would be favoured by natural selection, and all traces of former metamorphoses would finally be lost. If, on the other hand, it profited the young of an animal to follow habits of life slightly different from those of the parent-form, and consequently to be constructed in a slightly different manner, or if it profited a larva already widely different from its parent to change still further, 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 many animals. The adult might also become fitted for sites or habits, in which organs of locomotion or of the senses,

at a very early period of life, or each step might be inherited at an earlier period than that at which it first appeared. In either case (as with the short-faced tumbler) the young or embryo would closely 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 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,