A STUDY IN MORPHOLOGY. 
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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 recognise 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 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. 
When we compare man’s arm and leg we find that they have homologous features 
which are not only more recent than the time when man’s ancestors diverged from the 
ancestors of the birds, but more recent than the separation of the anthropoid and 
simian stems. They resemble each other in the texture of the skin and in the shape 
of the nails, and these resemblances are strictly homological, that is, they are not 
due to external conditions, but in spite of them ; and we meet with countless similar 
resemblances all through the animal kingdom. They are not accounted for by the 
“ metamere ” theory, even if this is fully accepted, for in many cases they are not old, 
but are of recent acquisition. 
In the case of the Crustacea the assumption that the remote ancestor of the group 
had a many-jointed body does not account for them ; and as the supposed necessity 
for an explanation of serial homology is the only reason for believing that this remote 
ancestor had a great number of body-segments, it is clearly illogical to reject the 
embryologies 1 evidence that this ancestor was a three jointed Nauplius, in order to 
hold an hypothesis which fails to account for the facts which are supposed to render it 
necessary. 
Explanation of the Plates. 
All the figures where the magnifying power is not stated were drawn with a power 
of 160 diameters (Zeiss, Oc. 1 , Obj. D) ; but the actual amplification of the drawings 
is not unilorm. In copying the original sketches it has been convenient to reduce 
the size of some of them, and no inference as to relative size should be drawn from 
any of them except where measurements are given. 
In order to render the figures as truthful and lifelike as possible, the animals were 
subjected to very little confinement while under examination, and as their incessant 
MDCCCLXXXII. * s 
