most in harmony with the surrounding conditions, it would maintain in the struggle for existence its characteristics against all tendency to vary in its offspring, which is equivalent to saying that it will remain unchanged so long as the environment remains the same. He then shows that in organisms in which the reproductive period covers many years, accelerated development by primogeniture, i.e., as between the first born and the last born of any pair and of their posterity, will, in time, produce differentiation. The series of the first born will, in the course of time, involve many generations at short distances from each other, whereas the series of the last born will, on the contrary, consist of a much smaller number of terms, each separated from its predecessor by a more considerable distance. Any tendency to variation from external or internal influences must needs find more numerous occasions to act in the series of the first born, not only because these have a more composite ancestry, but because they necessarily become the most numerous. In other words, the chances are more numerous for small differences among the first-born series, and, in proportion as such differences are accumulated, intercrossing with the series of the last born will become rarer. This law will gain from physiological selection, and, it seems to me, throws additional light on that of acceleration and retardation. It must act more particularly among higher animals, where the reproductive period is lengthened, and the time between the first and last born is great.
Saltation.—We are thus led to what have been called saltations in evolution. Although the history of palæontology has continually added to our knowledge of past forms, and helped to fill up many gaps in the evolutional series, and although during the last quarter of a century it has particularly vindicated Darwin's prophecy that many links would yet be found, the substantial truth remains that gaps still occur, and that progress, so far as present knowledge indicates, has been made by occasional saltations. There have been, it would seem, periods of rapid movement, and of comparative repose or readjustment of equilibrium. Cope concludes that "genera and higher categories have appeared in geologic history by more or less abrupt transitions or expression-points, rather than by uniform gradual successions."
One of Pictet's strongest points in opposition to Darwin's theory, which struck Darwin himself with much force, was that it ill agreed with the history of organisms with well marked and defined forms, which seem to have existed during but a limited period, as, for instance, the flying reptiles, the ichthyosaurus, belemnites, ammonites, etc. Some authors, who have fully recognized these gaps or leaps in the developmental history of animals, yet believe them to be consistent with the theory of gradual modi-
