A STUDY IN MORPHOLOGY. 129 



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 

 embryological 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 uniform. 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 



MDCCOLXXXII. S 



