38 Mr. E. R. Lankester on the use of 
lian heart; but since the common ancestor of mammals and 
birds in all probability had but three cavities to its heart, the 
ventricles are only homogenetic as a whole, and not each to 
each. The disposition of the aorta and the important light 
thrown on the origin of the muscular right auriculo-ventricular 
valve of the bird's heart by comparison with an Ophidian or 
Lacertian heart, harmonize decidedly with the conclusion that 
the right ventricle of the bird is not homogenetie with the 
right ventricle of the mammal. But it is said to be homo- 
? What is there more involved in the term 
r 
The common stock of these groups most 
, certainly had not such a specialization of this part of its mus- 
cular structures. What, then, is it that produces so close a 
resemblance in the disposition of these parts as to lead one to 
of homology? What is the other quantity covered by 
the term homology over and above homogeny $ 
The consideration of one more case, that of serial homolo- 
gies, will bring us to this: Unless it be maintained that the 
vertebrate animal is an aggregate of two individuals, one re- 
presented by the head and arms, the other by the legs, no 
genetic identity can be established between the fore and hind 
limbs. And since no one will maintain such a constitution for 
the Vertebrata (though it is exceedingly probable that the 
earliest segmentation which they exhibit 1s a remnant of such 
a history), the possibility of serial homogeny is out of the 
question in Vertebrata, though the segments of Arthropoda, 
Vermes, and other tertiary aggregates present it. And yet 
we speak of serial homologies; and it is possible to trace a 
very remarkable correspondence between the bones and mus- 
cles of the fore and hind limbs. What is the nature of the 
correspondence between fore and hind limb which is called 
“ serial homology ?” If we can ascertain this, we may expect 
to ascertain at the same time the nature of the correspondence 
