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542 these considerations in a masterly manner. But there was no time in the history of paleontology when they were not extremely obvious and familiar considerations. Chambers, in replying to his critics, had fallen back upon the argument from the inconclusiveness of negative evidence. Even Hugh Miller, without greatly profiting by his own precept, had pointed out "how unsafe it is for the geologist to base positive conclusions on merely negative data." And Spencer, in a brilliant article written in 1858, and published in the Universal Review in July, 1859, had urged that "along with continuity of life on the earth's surface, there not only may be, but must be, great gaps in the series of fossils;" and that "hence these gaps are no evidence against the doctrine of evolution." He concluded:

It must be admitted that the facts of Palaeontology can never suffice either to prove or disprove the Development Hypothesis; but that the most they can do is, to show whether the last few pages of the Earth's biologic history are or are not in harmony with this hypothesis.

In its later development, it is true, paleontology has been able to produce some striking supplementary evidences of evolution. In a limited number of cases, approximately complete and closely graduated series of forms of single orders or families can be exhibited in due stratigraphic superposition. But all the elaborate and impressive "form-series" have been worked out since 1859. Darwin himself made no original discoveries in this field; and as late as the sixth edition of the "Origin" the best evidence of the sort he presented from other writers is, I believe, summed up in these two sentences:

Several cases are on record of the same species presenting varieties in the upper and lower parts of the same formation. Thus Trautschold gives a number of instances with Ammonites, and Hilgendorf has described a most curious case of ten graduated forms of Planorbis multiformis in the successive beds of a fresh-water formation in Switzerland.

Of the two instances cited the first is vague—the great studies of Waagen (1869) and of Neumayr (1871-5) in the Ammonites were still to come; and the observations of Hilgendorf seem already, by the time the sixth edition of the "Origin" was prepared for the press, to have been shown to be erroneous. The best known example, to English readers, of a form-series is that of the Equidæ. But Rütimeyer's "Beiträge zur Kenntnis der fossilen Pferde" appeared only in 1863; and Huxley's researches in this field, which were the consequence, not the cause, of his acceptance of the theory of descent, were first presented to the public in his presidential address before the Geological Society in 1870.

6. The Argument from Persistent Types.—If good cases of