110 FACTOES OF EVOLUTION 



to the appearance, by comparatively abrupt transformations, of 

 multitudes of the lower forms of life such as Amcebae, flagellate 

 Monads, Moulds, Peranemata, Ciliated Infusoria, and many other of 

 the lower types which are by no means so dependent upon 

 specialised external conditions, and amidst which the part played 

 by ' natural selection ' seems reduced to its lowest terms.' 



Statements such as these are all that Prof. Weismann in the 

 course of eleven pages was able to say against the occurrence of a 

 per saltum development, as one of the modes in which Evolution is 

 carried on.= His statements and arguments must be absolutely 

 unconvincing to any open-minded critic. 



So far from there being any a priori objections to per saltum 

 development or Heterogenesis, which, as I shall claim to show, 

 occurs so frequently, the fact of its occurrence ought to be con- 

 sidered as thoroughly harmonious with all that we know concerning 

 allotropic and isomeric changes in simpler forms of matter, and 

 concerning the constitution of living matter itself. 



For the present, however, the subject of Heterogenesis is post- 

 poned, and I intend to limit myself to a brief account of the views 

 of de Vries. 



After pointing out that the origin of species has hitherto been 

 considered an excessively slow process and one "withdrawn from 

 actual observation, or at least from experimental treatment," he 

 contends that now in accordance with his views the process should 

 be regarded as much more rapid, and as one coming within the 

 range of direct observation and experiment. He disbelieves in 

 minute common variability as a starting point for natural selection 

 and the production of new species, and maintains that such a 

 process "does not lead by even the sharpest persistent selection, to 

 any real transgression of the limits of species, much less to the 

 origin of new and constant attributes." 



"According to the Mutation theory species have not ori- 

 ginated by gradual selection, continued through hundreds or 

 thousands of years, but by sudden steps, even if the changes are 

 very small. Unlike the variations which are progressive changes in 

 a straight line, those metamorphoses which are designated as 

 ' mutation ' branch off in new directions. Furthermore, so far as 



' In Chaps, ix-xiii. the origin of all these forms as a result of more or less 

 "abrupt transformation" will be dealt with, while many other heterogenetic 

 transformations are described in " Studies in Heterogenesis," 1904. 



= See his " Studies in the Theory of Descent," Transln., 2 Vols. 1882, 

 pp. 697-708. 



