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THE POPULAR SCIENCE MONTHLY. 



A, B, C, D, E, and F are descended. Even if it could be proved to 

 be the only species of its genus, and therefore the direct ancestor of 

 the recent species, the precise manner in which these are related to it 

 would still remain in uncertainty, for it may have given rise, at about 

 the same time, to two divergent variations, represented by the dotted 

 lines, and these may have led to the two forms a and b, each of which 

 gave rise to variations which resulted in the recent species A, B, 



Fig. i 



C, and D, E, F, or it may have persisted for a long time, and be- 

 come slowly modified, through the stages c, d, e, f, into the form g, 

 different from the form G, and still more different from the known 

 fossil M, before it gave rise to the ancestors of the two recent genera. 

 In this case the phylogenetic tree would be represented by the con- 

 tinuous light lines of the diagram. It is possible, again, that A, B, and 

 C are not equally related, but that two of them, B and C, for instance, 

 have had a common ancestor which was not in the line which led to 

 A, and in this case their relationships would be expressed by the 

 broken lines . . . of the diagram ; or, finally, it is perfectly 

 possible, and many naturalists think it is very probable, that evolution 

 has been guided by some more fundamental law than that of the 

 natural selection of divergent variations, and, if this is so, the charac- 

 teristics which distinguish A, B, and C may be due to something else 



