Origin of Species Variorum

Comparison with 1866

Text in this page (from paragraph 5000, sentence 400, word 12 to paragraph 5000, sentence 400, word 22) is not present in 1866

In two or more groups of animal, however much they may at present differ from each other in structure and habits, if they pass through closely similar embryonic stages, we may feel almost assured that they have descended from the same 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 all the developmental stages may have been suppressed, or may have been so greatly modified as no longer to be recognised, through adaptations, during the earlier periods of growth, to new habits of life. ↑ Community of descent will, however, often be revealed, although the structure of the adult may have been greatly modified and thus obscured; we have seen, for instance, that cirripedes, though externally so like shellfish, 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 shows us more or less completely the structure of their less modified ancient progenitors, we can .. see why ancient and extinct forms of life should resemble the embryos of our existing species, their descendants. 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 only in those cases .. in which the ancient state, now supposed to be represented in many embryos, has .. been obliterated neither by the successive variations ... having super- vened at a very early period of growth, nor by the variations having been inherited at an earlier period than that at which they first appeared. It should also be borne in mind, that the supposed law of resemblance of ancient forms of life to the embryonic stages of recent forms, may be true, but yet, owing to the geological record not extending far enough back in time, may remain for a long period, or for ever, incapable of demonstration. ↑

Thus, as it seems to me, the leading facts in embryology, which are second in importance to none in natural history, are explained on the principle of slight modifications not having appeared,

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, now supposed to be represented in existing embryos, has not been obliterated, either by the successive variations in a long course of modification having supervened at a very early age, or by the variations having been inherited at an earlier period than that at which they first appeared. It should also be borne in mind, that the supposed law of resemblance of ancient forms of life to the embryonic stages of recent forms, may be true, but yet, owing to the geological record not extending far enough back in time, may remain for a long period, or for ever, incapable of demonstration.

Thus, as it seems to me, the leading facts in embryology, which are second in importance to none in natural history, are explained on the principle of slight modifi- cations not appearing,