74 EVOLUTION OF TO-DAY. 



is duplicated. Supposing this division to continue, 

 we should soon get a chain of individuals which 

 grows longer and longer, and if the segments re- 

 mained connected, the result would be a segmented 

 animal, each segment of which is homologous with 

 all the others, for the simple reason of descent. 

 The serial homology of the appendages of such ani- 

 mals is, therefore, explained in a manner quite sim- 

 ilar to that of the first class. Whether this theory 

 is correct, is not definitely agreed upon by natural- 

 ists. It is true that such a division does take place 

 in many low animals. But it is also true that there 

 is no tendency shown to form thus single segmented 

 animals, but rather chains of independent individu- 

 als ; and further, the embryological evidence derived 

 from Crustacea is opposed to the view. 



But whether this theory of Spencer be true or not, 

 does not materially affect the argument we are con- 

 sidering, for it will not, does not, apply to the homol- 

 ogy of the four appendages of vertebrates. If ever 

 the vertebrates were derived from the division of a 

 primitive unsegmented animal, as above suggested, 

 it was very long ago, in fact before the animals pos- 

 sessed bones at all. Even supposing that the ap- 

 pendages did originally owe their likeness to such 

 descent, the homologies between the bones which 

 have since appeared must be a later acquired fea- 

 ture. The appendages must have grown alike. 

 Moreover, embryology has shown that the earliest 

 vertebrate appendages were very unlike the leg or 

 arm of man, and this is another proof of the second- 

 ary character of their present homology. Indeed, 



