Abbott's _Primitive Industry_, published in 1881, is a useful manual for studying this collection; and an account of his discoveries in the glacial gravels is given in _Reports of the Peabody Museum_, vol. ii. pp. 30-48, 225-258; see also vol. iii. p. 492. A succinct and judicious account of the whole subject is given by H. W. Haynes, "The Prehistoric Archaeology of North America," in Winsor's _Narrative and Critical History_, vol. i. pp. 329-368.] [Sidenote: The Calaveras skull.] But according to Professor Josiah Whitney there is reason for supposing that man existed in California at a still more remote period. He holds that the famous skull discovered in 1866, in the gold-bearing gravels of Calaveras county, belongs to the Pliocene age.[10] If this be so, it seems to suggest an antiquity not less than twice as great as that just mentioned. The question as to the antiquity of the Calaveras skull is still hotly disputed among the foremost palaeontologists, but as one reads the arguments one cannot help feeling that theoretical difficulties have put the objectors into a somewhat inhospitable attitude toward the evidence so ably presented by Professor Whitney. It has been too hastily assumed that, from the point of view of evolution, the existence of Pliocene man is improbable. Upon general considerations, however, we have strong reason for believing that human beings must have inhabited some portions of the earth throughout the whole duration of the Pliocene period, and it need not surprise us if their remains are presently discovered in more places than one.[11] [Footnote 10: J. D. Whitney, "The Auriferous Gravels of the Sierra Nevada", _Memoirs of the Museum of Comparative Zooelogy at Harvard College_, Cambridge, 1880, vol. vi.]