The Principles of Palaeontology. 1G9 



which, as we have seen, present the comparison of groups with 

 individuals. Yery many ancient forms have remained without 

 modification, so that if we confine our attention to the charac- 

 teristics of families or large genera, the ancient epochs are poor 

 in special types. The ancient types have indeed put forth many 

 progressive branches, but a vast number of their descendants 

 have remained without important evolution. There is then in 

 this nothing absolutely fatal, and the gradual improvement of 

 one form of a group in no way implies the disappearance of the 

 lower form. 



Another objection against the generality of this law is drawn 

 from cases of evident regression, which are so well known that 

 it is unnecessary to cite them in detail. The majority of para 

 sites are, in the adult state, in marked regression to a deter- 

 minate stage of their embryogenic development ; it is the same 

 with many attached animals, like the A-scidians. The regression, 

 moreover, is not in general anything more than the result of an 

 adaptation to a peculiar mode of life. 



The palaeontologic objection to the hypothesis of improvement 

 drawn from the simultaneous appearance of forms of unequal 

 grade in very ancient epochs, does not appear to us conclusive, 

 on account of the insuificiency of evidence concerning the 

 Silurian period. In fine, if we confine our attention to the grand 

 lines. Palaeontology on the one side elucidates the general law 

 that the most differentiated forms have almost always succeeded 

 the others, and on the other side it makes known the fact that 

 certain types have persisted without any important modification, 

 and that consequently improvement does not necessarily imply 

 the disappearance of the ancient forms inferior in organization. 

 Some forms then remain unchanged, but the greater number 

 evolve in a progressive direction. 



In order to explain this general tendency toward improvement 

 which thus manifests itself in all groups and affects all their 

 organs, some have thought it necessary to suppose an especial force, 

 a force innate in the living creature, a vital jphyletic force, the 

 effect of which would be precisely the gradual and final progres- 

 sion of the organisms which are derived one from the other. 

 The advantage of this theory is that it supplies a solution of the 

 difficulties which the doctrine of selection fails to solve, an. 

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