288 w. d. matthew phylogexy and correlation 



True Xature of a Phyletic Series 



In view of the above considerations, what is the real nature of a phy- 

 letic series, such as may be found in the successive geologic strata of any 

 region? Evidently its successive stages are not wholly in genetic se- 

 quence unless we are in the exact center of dispersal of the group. Xor 

 are they wholly successive immigrants arriving unaltered from that cen- 

 ter. Each stage is a complex and non-homogeneous group derived from 

 earlier and later strains of immigrants, modified to varying degrees by 

 the various environments through which they have passed. To disen- 

 tangle such a complex in any accurate and exact way is humanly impos- 

 sible, even where our material is abundant. Their approximate relations 

 may be presented in the form of a polyphyletic succession, but all such 

 subphyla must needs be more or less arbitrary and inexact. It is only 

 in the larger phyla that the arrangement accords with the real genetic 

 sequence. 



CORRELATION OF HOMOTAXIAL STAGES OF A PHYLUM 



There is one phase of this discussion that bears on the question of cor- 

 relation. It is customary to regard the appearance of equivalent stages 

 in a phylum in the formations of widely separated regions as the most 

 conclusive evidence of synchronism. This may be true where the regions 

 are equidistant from the center of dispersal of the group in question. 

 Where they are not, two considerations affect the conclusion. First, there 

 is the distance in time of migration of the race from the dispersal center 

 to the two regions compared, and this may be considerable, even in a geo- 

 logic sense. Second, there is the reactionary influence of previous stages 

 in the evolution of the group absorbed into the advancing new stage on 

 its way. This latter is more considerable in its influence than might at 

 first be supposed on species remote from their place of origin of the 

 group. By assuming certain special cases, it is possible to calculate what 

 •effect it has. 



In the accompanying diagram the horizontal distance left to right rep- 

 resents geographic migration from the center of dispersal. The vertical 

 distance from below upward represents geologic time-relations. A, B, C, 

 D, E, F represent successive geographic varieties or species, formed by 

 the influence of the local environments when the dispersal of the race 

 begins. 1, 2, 3. 4, 5, 6, etcetera, represent successive geological mutations 

 or phyletic stages in the center of dispersal. The upward trend of the 

 line A-F covers the time required for a migrating stage to reach the re- 



