80 THE THEORY OF EVOLUTION 



evidence of the transition of one family into another, 

 or one class into another, both in the animal and plant 

 kingdoms. Both must be considered in the same way, 

 unless the other indices for the evolution of organisms 

 comparative anatomy, embryology, animal and plant 

 geography, etc. lead to such strong indirect proof 

 that the negative results of palaeontology in separate 

 cases, or even generally, may be ignored. In this 

 case the theory or hypothesis could really serve as 

 an explanation of an occurrence without the occurrence 

 concerned being a support to the hypothesis. 1 



Then, for example, the conclusion that, despite 

 everything, a definite class must have been derived 

 from another, could, considered purely theoretically, 

 become an actual scientific postulate. 



The ' scientific postulates ' here the theory of evolu- 

 tion we meet with now very often in contemporary 

 literature. Many of these we cannot at all recognize 

 as such, since a condition absolutely essential for the 

 statement of a postulate is left out of consideration. 

 For instance, no trouble is taken at the outset to ascer- 



1 Only in this sense is Naegeli's expression correct : ' It was not my 

 intention to discuss all branches of the doctrine of descent . . . Therefore 

 the otherwise generally treated theme of geographical extension and the 

 palaiontological evidence have been almost entirely neglected by me, 

 because the existing demonstrated facts show themselves to be of manifold 

 significance, and because their explanation may be much rather expected 

 by means of a correct theory than that they should contribute appreciably 

 to the foundation of one.' (Meclianiscli-pliysiolog. Theorie der Abstam- 

 mungslehre, Munich and Leipzig, 1884, v.). Naegeli in point of fact pro- 

 ceeded in the said work on purely a priori lines, since what ' is ' he 

 spoke of as ' becomes,' expressing actual facts connected with the present 

 organisms in evolutional historical fashion. 



