CHAPTER XV. 

 THE QUESTION OF PHYLOGENY. 



As long as one accepted an uninterrupted progression 

 say from amoeba to man, it was justifiable to deduce 

 from the degree of similarity of a given form to the 

 amoeba, or to man, the place which this form ought to 

 occupy on the ladder leading from the amoeba to man. 



Of course, the stronger the belief in orthogenetic 

 evolution, the greater the degree of certainty with 

 which the place of a given form in phylogeny could be 

 determined. 



Some such degree of certainty could be attained in 

 Lamarck's evolutionary scheme, because this supposed 

 all progress to come about by response to necessity, 

 so that, if it was for-ordained that the amoeba should 

 better its constitution so as to reach that of man, there 

 was reason to suppose a rectilinear succession of types 

 also. 



Nageli's Vervollkommnungstrieb allows a similar 

 degree of certainty. This idea of a rectilinear progression 

 was given up by Darwin, who saw the cause of progres- 

 sion in the selection of the best of a number of allsi- 

 ded variations, comparing the road along which phylo- 

 geny proceeded, to a branched tree, in which the posi- 

 tion, to be assigned to a given individual, became very 

 doubtfull. 



Yet, as long as one believed similarity to be a measu- 



