MAIN PATHWAYS AND TENDENCIES 115 



roundings can be materially changed by dissolving 

 chemical salts not injurious to life in the water 

 inhabited by these organisms. Observations with 

 this kind of experimentation have been very helpful 

 in attacking problems of the life of organisms. 



Experiments on regeneration and conditions of 

 growth have been numerous, and experiments on 

 fertilization, both with sperms and with chemical 

 salts, are among those which have yielded important 

 results. In this connection should be mentioned the 

 experiments of Loeb on artificial parthenogenesis and 

 of Lillie on the nature of the fertilization process. 



Wilhelm Roux, who has done very notable work in 

 experimental morphology, has established a special 

 periodical (Archiv fur Entwickelungs Mechanik) for 

 the researches in this field. 



Philosophical Zoology. The profound changes in 

 thought produced by the establishment of the doc- 

 trine of organic evolution, and its effects upon our 

 interpretations of nature, gave rise to the division 

 of philosophical zoology. In this field are included 

 the rise of evolutionary thought and the formula- 

 tion of the different theories of organic evolution. 

 The latter are attempts to designate the physical 

 agency or agencies that have operated to bring about 



