and accessible. Except for the questionable attempts to estimate the size of the gene, the main conclusions concerning the mechanism of heredity and the orderly arrangement of specific genes in the chromosomes rest on quantita tive data and on analytical deductions that are tested by further experi ments wherever possible. The theory that the chromosomes carry the hereditary factors, and that these factors lie in linear order in the chromo somes, enables us to predict, with a high degree of

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... ainty, how any new character will be inherited with respect to all the other 300 known characters of Drosophila. If the accuracy of prediction is a test of the usefulness of a theory, then we may claim some justification for our view. More than this, I think, it is not necessary to claim for a scientific theory. ' The evidence has given us a glimpse at least of processes that are so orderly and so simple as to suggest that they are not far removed from physical changes ; and the order of magnitude of the materials is so small a8 to suggest that its component parts may come within the range of molecular phenomena. If so, we may be well on the road to the promised land where biological results may be treated as physical and chemical events. The Sympathetic Innervation of the Vagina.