290 W. D. MATTHEW PHYLOGEXY AND CORRELATION 



In other words, the successive remnants of the earlier stage absorbed 

 by mutation 1 on its journey from A to F have brought it nearly back to 

 the earlier phyletic stage, although the new stage displaced most of the 

 individuals of the earlier stage in each region it traversed. 



If now we calculate the affinities of the next phyletic stage in its 

 progress from A 2 to F 2 , we will find that 



B 2 = 3/4 A 2 + 3/16 A 1 + 1/16 A, 



and when we arrive at F 2 we find that the approximate percentages are 



A 2 = 24 per cent. 



A 1 = 29 per cent. 



A =13 per cent. 



AB = 12 per cent. 



. ABC = 10 per cent. A and modifications, 49 per "cent. 



ABCD == 8 per cent. 



ABCDE == 6 per cent. 



In terms of structure this means that the representatives of the third 

 phyletic stage by the time they reach region F will have reverted so far 

 as to be less advanced than the second stage, and also undergone a large 

 amount (36 per cent) of atypical modification from the intermediate 

 environments. 



If we were correlating the fauna of region A with the fauna of region 

 F on the usually accepted basis of identical or equivalent species, we 

 should conclude that the appearance of F 2 . which is between A and A 1 in 

 its structural progressiveness. should be correlated with strata in region 

 A between A and A 1 . In point of fact it is later than A 2 by the amount 

 of time required for a migrating stage to traverse the distance A to F. 



CORRELATIOX OF CeXOZOIC MAMMAL FaEX.E 



These considerations have some bearing on the correlation of Tertiary 

 horizons in Europe and the United States : but so much of our mam- 

 malian fauna is derived from dispersal centers more or less equidistant 

 from these two regions that no serious error can ensue from neglecting 

 them. The case is somewhat different when we correlate the faunae of 

 the southern continents with those of the Holarctic realm. In these we 

 should evidently make some allowance for the time required to reach 

 regions remote from the dispersal center and for the reactionary effect of 

 absorption of more primitive stages. Both will tend to make southern 





