CREATION BY EVOLUTION 



form M. corbovis. One kind retained the flat top and elon- 

 gated form of the typical corbovis; another became arched 

 above ; a third developed a prominent posterior ridge ; and a 

 fourth became conical or pyramidal. These four series may 

 all be included in the same species, being, according to Poul- 

 ton's term, epigonic — that is, descended from one pair of 

 ancestors; or each series would answer to Darwin's concep- 

 tion of a species, for each showed an appreciable difference 

 and persisted for a considerable period. Rowe's collection, 

 however, showed that though the different shapes were devel- 

 oped by slow change, they were not developed by continuous 

 divergence from the original type. The contemporaries of 

 the early corbovis included dome-shaped and flat varieties. 

 The forms of Micraster found at each horizon or each level 

 in the Chalk show variations that were probably dependent 

 on the nature of the sea floor and on the movement of the 

 sea water. The evolution of the four chief varieties of 

 Micraster in the Upper Chalk was not due to steadily pro- 

 gressive variation from the ancestral type but proceeded by 

 innumerable minor irregular variations, the effect of the 

 environment acting upon successive multitudes of Micraster. 

 The evolution was doubtless due to syngamy or interbreed- 

 ing under natural conditions, with the encouragement by the 

 environment of the useful modifications. It did not proceed 

 along lines of continuous and steadily diverging variation, 

 but along several parallel lines, from each of which there 

 were variations that would overlap those from the next line. 

 Dr. Rowe, in his diagram showing the evolution of the Chalk 

 Micraster, used the plan of branches divergent like the twigs 

 of a tree. The main types were established by parallel lines 

 of descent, the variations from each line being radial in all 

 directions and overlapping those from adjacent lines. This 

 conception agrees with that stated by Poulton when, in 



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