126 
MR. W. K. BROOKS OK LUCIFER: 
dualities of the somites or metameres, and by the increased structural and functional 
specialisation and differentiation of each appendage as compared with the others. 
This series of changes is so well exemplified by the study of adult and larval 
Crustacea; it is so remarkable and interesting; so very conspicuous and unquestion¬ 
able, that it has long attracted the attention and called forth the speculation of 
morphologists. It is natural to suppose that the process of change which is open to 
our observation through study and comparison of living Crustacea, is a continuation 
of a, similar process which went on in the remote past. There seems then at first 
sight to be reason for believing that, if we could go far enough back, we should find 
the individuality of the whole organism gradually disappearing and giving place to 
the separate individualities of the component somites; that we should find the 
specialisation of the appendages gradually disappearing, until we should at last find, 
as the remote ancestor of the Crustacea, a series or community of independent 
organisms, each one essentially like the others, and able to provide for its own wants 
and to lead an independent existence when accidentally or naturally detached. 
This view has been advocated at length by ELeckel Generelle Morphologic,’ 
1866 ) and by Spencer (‘Principles of Biology,’ vol. xi., 1867 ), and used by both 
these writers as an explanation of the origin of all segmented or compound animals 
and plants. It has been accepted, with more or less qualification, by many other 
writers, although Huxley (‘ Oceanic Hydrozoa ') and Metschnickoff (Zeit. f. Wiss. 
Zool., xxiv.) have pointed out that, even in the Siphonophorse, where the individualities 
of the units in the compound are extremely well marked, the view that the organism 
has been evolved by the gradual integration and specialisation of originally inde¬ 
pendent Zooids is attended with serious difficulties. 
So far as we can see there is no reason why the Crustacea might not have originated 
in this way, by the gradual integration and differentiation of a community of inde¬ 
pendent metameres, but the evidence which is attainable seems to directly oppose the 
belief that this has actually happened. We are able to trace the higher Decapods 
back, very satisfactorily, to a Phyllopod-like ancestor with a long series of undif¬ 
ferentiated somites and appendages, but even here the somites are simply parts of 
the body, and they furnish no more evidence than those of a Crab to show that they 
ever were the independent organisms of a community. 
When we attempt to go still further back we find that the facts of embryology, if 
they show any thing whatever about the phyllogeny of the Crustacea, lead us back to a 
Ncmplius with three inter-dependent somites and three pairs of specialised appendages, 
rather than to a form with a great number of unspecialised somites and similar 
appendages. 
Turning now to a somewhat different aspect of the subject, we notice that, if we 
confine ourselves to structure, and leave out of sight the question of origin, there is 
the closest similarity between serial homology and the homology between the corre- 
