570 THE HISTORY OF BIOLOGY 



distress at these misfortunes is said to have broken the aged patriot and 

 philanthropist and to have appreciably shortened his life. 



Eimer's "orthogenesis" theory 

 We shall mention in the following some further tests of the descent theory, 

 though more as proofs of the increasing difficulties with which the successors 

 of Darwin had to contend than for any actual value that the results possessed. 

 Theodor Eimer (1843-98), a fellow-countryman and disciple of Kolliker's 

 and eventually professor at Tubingen, endeavoured to solve the difficulty of 

 the descent theory chiefly along the lines laid down by Nageli. He rejects 

 Darwin's theory of variation in all possible directions as the basis of selec- 

 tion; the development of the organic forms must rather, he holds, depend 

 upon a force operating in a definite direction, a force induced and modified 

 by outward influence, such as light, air, heat, nourishment, and thus one 

 that provides selection with the material for changes which it influences. 

 This definitely directed evolution he terms "orthogenesis" and he seeks 

 to prove that it is indispensable for the building up of species; selection 

 alone cannot produce anything new, but rather it is this inner force, law- 

 bound, yet aff"ected by external influences, that is the true origin of life- 

 forms. In proof of this he cites a large number of observations dealing with 

 the colour changes in butterflies, as well as the development of the skeleton 

 of vertebrates, which attracted great attention at the time and brought their 

 originator a large following, but which are now no longer up to date. 



Semon s " mneme" theory 

 Richard Semon (i 859-1919), a pupil of Haeckel and at one time a professor 

 at Jena, pursued another line of thought. His theory of evolution is also 

 based entirely on the belief in the transmission of acquired characters, but 

 he endeavours to give this theory a direction more suited to the time by 

 submitting it in a new form. To his mind, the weakness underlying previous 

 theories of this kind had been due to lack of clearness in the term "char- 

 acter"; instead of transmission of acquired characters it should be called 

 transmission of acquired reactions. For the hereditary transmission depends 

 upon the nature of the germinal plasm, and this reacts under natural laws 

 to the influence of the general condition of the body. The power of living 

 substance to react, its " Rei^barkeit," is the primary cause of evolution. Even 

 if the external influence upon it is transitory, there nevertheless remains an 

 impression, which becomes a decisive factor in its future development. This 

 impression is termed " Engramm." And the altered construction of the new 

 generations' form, upon which natural selection has an extenuating influence, 

 is due to the co-operation between the outwardly induced Engramm of the 

 body and that o the germinal plasm. This influence of the corporeal sub- 

 stance upon the germinal plasm is called "somatic induction," and the as- 

 sumed power of the living substance to preserve external impressions, upon 



