OTHER THEORIES OF EVOLUTION 325 



but they are akin to von Nageli's and those of certain later 

 authors in their recognition of an internal growth-force. 



The idea of a determinate evolutionary path traversed 

 by a group of animals without reference to Natural Selection 

 has been adopted by a large number of authors, some of them 

 previously to Eimer, and by some without any acceptance 

 of the belief that the directive force is environmental. Hyatt 

 (1894), Gadow (191 1 ), Dunbar (1924), and Berry (1928) are 

 exponents of Eimer's view. A number of palaeontologists 

 insisted on the determinate nature of certain evolutionary 

 series without committing themselves to any causative agency. 

 ' Determinate ' series have been noted in the Opalinidae and 

 Salpidae (Metcalf, 1928), Pigeons (Whitman, 191 9), Garter 

 Snakes (Ruthven, 1908), Beetles (Kellogg, 1906), and other 

 groups. Two particular aspects of this ' determinate ' evolu- 

 tion have been made special subjects of study and theory : 

 (1) The progressive attainment of monstrous size, either of 

 the whole individual or of a part (' Momentum ' (Dendy), 

 ' Hypertely ' (Cuenot), ' Disharmony ' (Champy) ). (2) The 

 phenomena of recapitulatory series involving changes of a 

 degenerative or ' senescent ' type have been the source of 

 much study and speculation by the students of many groups 

 (Ammonites, Brachiopods, Reptiles), and a particular aspect 

 (the development and modification of spines) has been fully 

 studied by Beecher, who has described senescent types of 

 spine-formation in a great variety of groups. Analogous 

 cases are found in the histories of ornaments and septa in 

 Ammonites. 



A good review of the majority of the phenomena that have 

 at one time or another been treated as examples of determinate 

 evolution is given by Fenton (1931), though his survey does 

 not include a consideration of heterogonic growth and of 

 excessive size in general. 



It has often been urged that Orthogenesis is merely a 

 term by which we designate certain kinds of evolutionary 

 phenomena and that it does not involve any explanation of 

 them. Whether his theory is valid or not, Eimer did in fact 

 apply the term to a causal principle. Other writers have 

 used it to designate certain evolutionary events for which 

 they fail to find a satisfactory explanation in other theories 

 and which, by implication or otherwise, they attribute to 



