oj- the Vertehi-ate Skeleton. 43 



are products of divine law, and in a conviction of duty to seek 

 out its working" in all Avays. 



The scheme of the skeleton now sketched is what may be 

 named a potential skeleton ; and whatever value it has is in 

 the insight it gives into the relations to each other of the parts 

 of skeletons and the importance of resemblances between 

 similar parts in different skeletons as evidence of genetic rela- 

 tion. All the types of vertebrate animals are based upon this 

 general plan, and each ditfers from the other in some compa- 

 ratively slight details of potential growth ; and there is no- 

 thing peculiar in the genera referable to each of these minor 

 types except a varying growth, or suppression of growth, or 

 combinations of growths in the difierent bones of the body : 

 such modifications are the kinetic skeleton. If we find simili- 

 tudes between bones when they are compared together, the 

 comparison becomes meaningless and unprofitable unless we 

 believe the similitudes to be consequences of laws which can 

 be traced in their effects. The idea of affinity expresses faith 

 in such laws by teaching that the structural resemblances be- 

 tween animals are a consequence and evidence of an original 

 community of plan now onl}^ seen in fragments. And an 

 original common plan for vertebrates, a potential skeleton, 

 implies that the physical laws of nature producing growth 

 have upon their simpler product acted in differing ways, so 

 that the energy of the type became manifest in the divergence 

 of special different parts which make the plans of the several 

 vertebrate classes. 



Hence the practical question, affecting all comparative 

 study, after the mind has cancelled whatever osteological 

 structures are variable in the type (and therefore demonstrably 

 kinetic), is to discover in what direction each order has di- 

 verged from the common plan, and in what way this diversity 

 obscures or renders clear its affinities with the other orders. 

 To put a special case : — in what direction relatively to the 

 vertebrate common plan is the osteology of a tortoise deve- 

 loped ? and how far from this osteology can we infer a com- 

 munity of divergence between the tortoise and all other or 

 any other known animals ? Those points of divergence in the 

 potential skeleton would be the osteological affinities of an 

 animal, and, determined for a number of known types, would 

 enable us to predicate within approximate limits the characters 

 of many extinct orders of which the existence is at present 

 hardly suspected. 



To examine such a problem, it is necessary to be familiar 

 with the facts which are factors in it ; and so to these we 

 must next turn. 



