MODERN BIOLOGY 555 



also take in conjunction with this theory his positive assertion, previously 

 referred to, as to the pleomorphism of micro-organisms; it cannot, of course, 

 be denied that the absence of any species-characters in them was a feature 

 of primitive organization, which would make them closely akin to lifeless 

 matter. 



His descent theory 

 Nageli's descent theory is also in line with his theory of spontaneous genera- 

 tion; since spontaneous generation goes on incessantly, it is possible to sup- 

 pose that the most highly developed organisms are really the oldest, while 

 the primitive organisms have been evolved later. His descent theory has thus 

 acquired a decidedly polyphyletic character, and, strictly speaking, it does 

 not presuppose any transition from one species to another. The most inter- 

 esting feature in Nageli's phylogenetical speculation — recorded in an essay 

 on the genesis of the natural-historical species (1865) and in a large work, 

 Mecbanisch-pbysiologische Theorie der Abstamtnungslehre (1884) — is his criti- 

 cism of Darwin's theory of selection. In the course of his experiments on 

 the Hieracium he had discovered that the external conditions of life which 

 cause the struggle for existence do not alter the life-types; species that are 

 placed in new conditions of life do not assume any similarity to kindred 

 species previously brought under these conditions. Natural selection, there- 

 fore, cannot possess any form-building power; it does exist, but has only an 

 extenuative effect on middle forms. Instead, evolution takes place out of the 

 inner being of the organisms in virtue of an internal force, which Nageli 

 most frequently terms " Vervollkommmtgskraft," and once, in imitation of 

 Blumenbach, " Nisus formativus," a force by means of which the development 

 is led in a certain direction, not, as Darwin holds, to variations in every 

 possible direction. This force, however, is by no means a special life-force; 

 on the contrary, it is compared with the inertia in inorganic nature; just 

 as a rolling globe goes on until it meets an obstacle, so organic evolution — 

 it, too, being a movement — advances until an obstacle comes in the way. 

 These obstacles can be either the struggle for existence, or else direct ma- 

 terial influence due to irritation; according to Nageli the ruminants have 

 got horns as a result of striking their foreheads against one another — an 

 explanation in the spirit of Lamarck. 



His heredity theory 

 In connexion with his doctrine of descent Nageli propounds a heredity the- 

 ory of his own. At variance with Haeckel's view on the undifferentiated 

 character of the egg-cell he definitely maintains that the egg-cell is as com- 

 plicated as the creature which is to be evolved therefrom; the qualities of 

 the coming individual are all united in the egg-cell. But because this latter 

 and the sperm cell, in spite of their difference in size, have an equally large 

 share in the qualities of the new individual, these qualities cannot be 



