166 THE THEORIES OF EVOLUTION 



nature has not been determined yet, and which is 

 called biotactism, a certain morphogenic action is 

 exerted by known factors, heat, light, electricity, 

 gravity, fluids, pressure, etc. Numberless books and 

 pamphlets have been published in the past twenty 

 years demonstrating the importance of those factors 

 in ontogenetic development. A special periodical 

 founded by Roux, the Arcliiv filr Entwickelungsme- 

 chanik, is devoted to this branch of biology which has 

 been called the mechanics of development or, better 

 still, biomechanics. 



The various contributions to the study of the sub- 

 ject are not all inspired by a common theoretical 

 thought, nor are all their authors exponents of the 

 organicist theory. Herbst, for instance, endeavours 

 to make his conclusions harmonise with Weismann's, 

 a rather hopeless task, and Hertwig, the foremost 

 exponent of the epigenetic school, has even formulated 

 a theory of his own, his theory of idioblasts, particles 

 which determine the elementary properties of the cells. 

 This theory is an adventitious part of his system and 

 adds very little to his explanation of ontogenesis. 



Roux, whose theory attaches so much importance to 

 the influence of activity upon the formation of organs, 

 shows himself less uncompromising than Hertwig 

 when it comes to the question of preformation and 

 ontogenesis. According to Roux, the nucleus of the 

 ovum is made up of materials, qualitatively different, 



