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Show 1884.] OF THE INDIVIDUAL AND OF THE SPECIES. 471 first double, and that the progress of centralization suppressed one side of each metamere as the community became gradually fused into a bilateral organism, we may make the same statement regarding symmetry. " A process of evolution of this sort is not impossible .... The Salpa-chain is a bilateral community, and in Doliolum we have a similar community which exhibits considerable polymorphism. If this process were carried a little further, we might ultimately have a bilaterally symmetrical organism in which corresponding parts in the series or on opposite sides should be strictly homologous by descent; but we are not therefore justified in assuming that all instances of serial and lateral homology have originated in this way, and even if we were, a more careful analysis will show that the assumption does not remove all the difficulties. If we grant, for the sake of argument, that the Crustacea are not the descendants of Nauplius, but of a remote ancestor which consisted of a community of independent metameres, we shall still bs forced to recognize a bond of relationship between the limbs of a Decapod, which is very much more recent than that which they owe to common descent from the parent of the group of Zooids which formed the ancestral community. "The first, second, and third thoracic limbs of the adult Lucifer agree with each other, or are homologous, in certain features which are not present in a Schizopod. The exopodite is absent and the endopodite is long and slender in all of them, and it carries short hairs along its entire length, while in the Schizopod-larva the exopodite is present and the long hairs are restricted to the tip of the stout endopodite. W e must therefore recognize a bond of union or homology between these three appendages which has determined that they shall be like each other in the adult Lucifer ; and the assumption that this similarity is due to heredity from the parent of the imaginary metameres which joined together to form the primitive Crustacean, is out of the question, for we know that no further back than the Schizopods these appendages had quite a different structure. "The study of serial or lateral homology in other groups of animals forces us to the same conclusion, and compels us to recognize a persistent bond of union between them which cannot be due to what we usually understand by heredity. " O n the assumption that the Vertebrates are the descendants of a community of metameres, the genetic relationship between a Man's arm and a Bird's wing must be almost infinitely closer than that between a Man's arm and his leg, and this again much more recent than that between his right and his left arm. The arm and wing inherit their homology from the anterior limb of the common ancestor of M a n and the Birds ; but Man's arm and leg have no common ancestor more recent than the limb of the parent of the imaginary metameres which gave origin, by their union, to the ancestor of the Vertebrates, and the common ancestor of the right and left arms must have been still more remote. " W h e n we compare Man's arm and leg we find that they have |