562 THE CAUSES OF EVOLUTION 



theory 'of evolutionary causation. The Lamarckian Theory 

 supposes such variations to be induced by use, disuse, and environ- 

 ment. The Theory of Natural Selection assumes the appearance of 

 such heritable variations without attempting to explain their 

 causation. The Mutation Theory distinguishes between fluc- 

 tuations, which are not inherited, and mutations, which are inher- 

 ited and may furnish material for evolution. It does not neces- 

 sarily explain the causes of mutations. It is true that de Vries 

 put forth the theory that species have mutating periods extending 

 over many generations, but he offered no adequate explanation 

 of the occurrence of such peiiods. Orthogenetic theories attempt 

 to explain why heritable variations occur; but, more than this, 

 they postulate that certain kinds of variations are destined to arise 

 and hence to determine the course of evolution. 



Mechanistic Orthogenesis. — Orthogenetic theories that are 

 mechanistic suppose variations to be restricted, and evolution 

 thus directed, to certain lines, by the chemico-physical nature 

 of the organism and its environment. Cases are believed to exist 

 in which the fossil record suggests a determinate course of evolution 

 {cf. Fig. 275, p. 514). Examples are found among the dinosaurs (c/. 

 Fig. 269, p. 506), in which horns first appear as protuberances that 

 are hardly distinguishable, and certainly not useful, and increase as 

 though destined to a certain course. Again, many fossil series 

 suggest that types begin simply, and evolve complexities, such as 

 over-developed spines and plates, which foreshadow extinction. 

 Isolated examples of this would not be significant, but some paleon- 

 tologists claim that changes of this general sort have occurred so 

 often in wholly different types of animals that they can only be 

 explained as the result of forces inherent in the race and carrying 

 it along a " straight " or orthogenetic line of evolution. Other 

 evidence of orthogenetic evolution is found, by advocates of this 

 theory, in Hving animals; as in the studies of Professor Eimer upon 

 color markings of Uzards, beetles, butterflies, molluscs, etc. Ac- 

 cording to the mechanistic concept, all such evolution is due to 

 unknown but determinate physico-chemical factors. 



Vitalistic Orthogenesis. — On the other hand, orthogenesis may 

 be conceived as due to a vital force inherent in protoplasm and 

 determining aUke its immediate responses and the course of its 

 evolution. In criticism of all theories of orthogenesis, it can be 

 said that they are more philosophical than scientific. For all we 



