CREATION BY EVOLUTION
form M. corbovis. One kind retained the flat top and elongated 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 Poulton’s term, epigonic—that is, descended from one pair of ancestors; or each series would answer to Darwin's conception 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 developed 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 progressive 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 interbreeding 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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