Origin of Species Variorum

Comparison with 1872

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. 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. ↑ 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 to follow habits of life in any degree different from those of their parent, and consequently to be constructed on a slightly different plan, or if it profited a larva already different from its parent to change still further, 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 in the larva might, also, become correlated with successive stages of its development; so that the larva, in the first stage, might differ greatly from the larva 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

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. ↑ 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. Again, with respect to the singular fact of so many terrestrial and fresh-water animals not undergoing any metamorphosis, whilst marine members of the same groups pass through various transformations, Fritz Müller has suggested that the process of slowly modifying and adapting an animal to live on the land or in fresh water, instead of in the sea, would be greatly simplified by its not passing through any larval stage; 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. 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, &c., would be useless; and in this case the .. metamorphosis would be ...