OTHER THEORIES OF SPECIES-FORMING. 289 



I am not mistaken in this, the reconciliation for natural 

 selection and orthogenesis is at hand" (p. 4). 



In the category of determinate or orthogenetic variation 



should be included Delage's 24 not very clearly distinguished 



variation venerate. "We call by the name of 



Delage'a the- 



ory of general general variation," he says, "that which appears 

 variation. ^ Qne fj me m a ]j ft\e individuals of a race or 



at least a large number of individuals and which affects, 

 most often, several characteristics, if not all, in various 

 degrees of strength. Variations of this sort must be due 

 to modifications of the germ plasm produced either by the 

 reducing division, or by fertilisation, or by accidental altera- 

 tions which this plasm undergoes in its various divisions." 

 It is to these variations, according to Delage, that species- 

 change is due. The inducing influences which result in the 

 appearance of general variations are use and disuse and the 

 "conditions of life" (nutrition and climate). 



As forming a sort of link between the theories of ortho- 



genesis and those of heterogenesis (to be discussed in the 



next chapter), may be mentioned the rather 



-theory of vague and unformed theory of Jaeckel, 25 the 



metakinesis, Berlin pa i^ onto i ogist) called "metakinesis." 



Taeckel believes, from a study of fossil animal^series, that 

 there exists evidence of orthogenetic descent, but that while 

 genera and families may show continuous phyletic series, 

 speries" j appi5af sporadically, suddenly, and without special 

 reference to the phyletic series. He believes that many cases 

 of epistasis occur: that is, that many sexually mature ani- 

 mals show arrests of development in early ontogenetic 

 stages, and have therefore given up a former further 

 development. He finds numerous examples of this condi- 

 tion among fossil crinoids and trilobites and living sela- 

 chians. What his theory of metakinesis really seems to be 

 is a combination of the sudden, definitive appearance of new 

 species, which is the essential conception in the theories of 



