44 



On the Origin of the Vertebrate Skeleton. 



Tlie correspondence of parts is frequently close between 

 animals which would not be placed by classifiers in the same 

 natural group ; so that, as animals can only have diverged in 

 many different directions, or in directions Avhich are approxi- 

 mately parallel, it is impossible not to believe that the corre- 

 spondence is the evidence of some kind of parallel relation 

 between the groups, which may, of course, be a parallel func- 

 tion kinetically modifying different common plans, or parallel 

 plans kinetically modified by different functions. Each verte- 

 brate class consists of orders, but if these are arranged in 

 sequence of classificational semblance, their bones do not 

 graduate from one group into another: the lowest mammal 

 does not graduate into the highest bird, nor is there a sequence 

 from the bird down to the reptile. Classifiers, however, have 

 always agreed that there is something unnatural in the best 

 grouping according to a logical system, because it removes 

 from near association animals which have real affinity with 

 each other. Nor can this be surprising, when we remember 

 tliat by a class of animals is practically understood a certain 

 horizon or grade of complexity of soft structures. So that if 

 the organization of the bird, for instance, has any relation of 

 affinity with mammal or reptile, the relation must be with 

 some specified order of reptile or mammal, and must be due 

 to their all having diverged in the same direction from the 

 common plan, all being the consequence of a line of variation 

 which has preserved parts of the skeleton unaltered for them 

 all, while the soft parts have become more and more complex, 

 in such ways that the ordinal stem has been divided at inter- 

 vals into parts which are successively named, it may be, fisli, 

 reptile, and bird. If there is foundation for such a view, there 

 can be no such close osteolo- 

 gical resemblance between the 

 different natural groups of ani- 

 mals upon the same horizon of 

 organization as there must be 

 between some animals upon 

 that horizon and some animals 

 upon another horizon. This 

 proposition may be exempli- 

 fied by a diagram of a hand, 

 where there may be supposed to 

 be five stems, springing from 

 a common plan, and it might 

 be better exemplified by taking 

 the entire limb as a ty])c, where tlie humerus would stand for 

 the connnon plan. Such a diagram expresses the idea that 



