OTHER THEORIES OF SPECIES-FORMING. 279 



tions; they rest on the assumption that physico-chemical 



factors produce direct effects on the plastic organism, and 



that such effects, repeated and intensified, re- 



Another type 



f orthogenetic suit in a certain degree of modification or con- 

 trol of variation and evolution. To be sure, 

 there is not yet proposed a very satisfactory mechanism 

 for conveying the environmental influence to, and trans- 

 lating it into definite effect on the course of development, 

 but the obvious fact that environment does strongly affect 

 and modify individual function and structure and the rea- 

 sonable belief that the modification of the race must ulti- 

 mately rest on and proceed from the modification of the 

 individual, make the theories of orthogenesis based on en- 

 vironmental influence very suggestive and of distinct 

 scientific value. In addition, too, there is a certain amount 

 of actual evidence of observation for orthogenesis : an 

 evidence of two categories, namely, positive affirmative 

 evidence, 14 and negative evidence drawn from the inade- 

 quacy of other theories, notably natural selection, to explain 

 certain observed phenomena which can be explained on the 

 assumption of an orthogenesis. The general character of 

 this evidence is indicated in our first paragraph treating of 

 orthogenesis. To this may be added an ab- 

 stract of Plate>s " r "me of the facts or 

 to prove ortho- phenomena which may be looked on as positive 

 evidence for orthogenesis (although Plate cau- 

 tiously notes that some of these may be only phenomena of 

 orthoselection). These phenomena pointing toward ortho- 

 genesis may be grouped into six categories : 



i. The ''analogous or parallel variations" which have 

 been recognised ever since Darwin's time, he, himself, list- 

 ing many examples of them. These are varia- 

 tions of u nmi stakably similar character, which 

 often appear in different branches of the same 

 large group. "Comparative anatomy reveals many ex- 



