128 MB, W. K. BROOKS ON LUCIFER: 



the phylogenetic point of view it is a resemblance which is due to community of origin 

 or heredity from a common ancestor, while the differences between homologous 

 organs are due to the divergence of allied forms, and to the selection and perpetuation, 

 through natural selection, of variations which are in accordance with changed conditions 

 of life. 



Now are the phenomena of serial and lateral hornology like those of special homology 

 in this second or phylogenetic sense, as well as in a morphological sense ? 



On the assumption that the remote ancestor of the Crustacea was a community of 

 independent organisms, all of which had inherited their organisation from the same 

 parent, we might answer that serial homology is like special homology when viewed 

 from a phylogenetic stand-point, and if we assume that this series was at first double, 

 and that the progress of centralisation 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, and in some cases there seems 

 to be evidence that it has actually occurred. Pyrosoma is clearly a community of 

 independent Ascidians, which has been brought by natural selection into a form which 

 has a certain degree of individuality of its own, independent of that of the component 

 units ; although in this case the peculiar form of the community has called for little 

 differentiation, and the polymorphism is therefore very slight. 



The salpa-chain is a bilateral community, and in Dolioluin 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 a Nawplius, but of a remote ancestor which consisted of a community of independent 

 metameres, we shall still be forced to recognise 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. 



A reference to the figures will show that 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 recognise 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 assumption that this similarity is due to 

 heredity from the parent of the imaginary metameres which joined together to form 



