1884.] OF THE INDIVIDUAL AND OF THE SPECIES. 4/1 



first double, aud that the progress of centralization suppressed one 

 side of each metainere as the community became gradually fused 

 into a bilateral organism, we may make the same statement regard- 

 ing 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 

 serits 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 v/e grant, for the sake of argument, that the Crustacea are not 

 the descendants of Nauplius, but of a remote ancestor which con- 

 sisted 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. We must therefore recognize a bond of union or homo- 

 logy between these three appendages which has determined that 

 they shall be like each other in the adult Lucifer ; and the assump- 

 tion 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. 



" On 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 Man and the Birds ; but Man's arm and leg have no 

 common ancestor more recent than the hmb of the parent of the 

 imaginary metameres wliich gave origin, by their union, to the 

 ancestor of the Vertebrates, and the common ancestor of tlie right 

 and left arms must have been still more remote. 



" When we compare Man's arm and leg we find that they have 



