ON THE CAUSES OF VARIATION. 817 



most in harmony with the surrounding conditions, it would main- 

 tain 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 repro- 

 ductive 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 differentia- 

 tion. 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 addi- 

 tional 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 palgeontology has contin- 

 ually 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 salta- 

 tions. 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, be- 

 lemnites, ammonites, etc. Some authors, who have fully recog- 

 nized these gaps or leaps in the developmental history of animals, 

 yet believe them to be consistent with the theory of gradual modi- 



TOL, XXXIT. 52 



