523 



That is to say, alike in ascending through the Vertebrata gene- 

 rally, and in tracing np the successive phases of a mammalian em- 

 bryo, the last-developed and least constant division of the limb, is 

 that basic one by which it articulates with the hsemal arch. It 

 seems to us that, so far from proving his hypothesis, Professor 

 Owen's own facts tend to show that limbs do not belong to the 

 vertebrae at all ; that they make their first appearance peripherally ; 

 that their development is centripetal ; and that they become fixed 

 to such parts of the vertebrate axis as the requirements of the case 

 determine. 



But now, ending here this digressive exposition and criticism, 

 and granting the position that limbs " are developments of costal 

 appendages," let us return to the question above put Why are not 

 these appendages included as elements of the " ideal typical ver- 

 tebra 1 " It cannot be because of their comparative inconstancy ; 

 for judging from the illustrative figures, they seem to be as con- 

 stant as the haemal spine, which is one of the so-called autogenous 

 elements : in the diagram of the Archetypus, the appendage is re- 

 presented as attached to every vertebrate segment of the head and 

 trunk, which the ha3mal spine is not. It cannot be from their com- 

 parative unimportance ; seeing that as potential limbs they arc 

 essential parts of nearly all the Vertebrata much more obviously 

 so than the diapophyses are. If, as Professor Owen argues, " the 

 divine mind which planned the archetype also foreknew all its 

 modifications ;" and if, among these modifications, the development 

 of limbs out of diverging appendages was one intended to charac- 

 terize all the higher Vertebrata; then, surely, these diveTging ap- 

 pendages must have been parts of the " ideal typical vertebra." 

 Or, if the " ideal typical vertebra" is to be understood as a crystal- 

 line form in antagonism with the organizing principle ; then why 

 should not the appendages be included among its various offshoots ? 

 We do not ask this question because of its intrinsic importance. 

 We ask it for the purpose of ascertaining Professor Owen's method 

 of determining what are true vertebral constituents. He presents 

 us with a diagram of the typical vertebra, in which are included 

 certain bones, and from which are excluded certain others. If re- 

 lative constancy is the criterion, then there arises the question 

 What degree of constancy entitles a bone to be included ? If re- 

 lative importance is the criterion, there comes not only the question 

 What degree of importance suffices ? but the further question 

 How is importance to be measured ? If neither of these is the 

 criterion, then what is it ? And if there is no criterion, does it 

 not follow that the selection is arbitrary ? 



This question serves to introduce a much wider one : Has the 

 ideal typical vertebra " any essential constituents at all ? It might 



