ELEMENTARY VITAL PHENOMENA 187 



All living substance, like every physical body, must have some 

 form, which is determined by its relations to the chemico-physical 

 conditions of its environment. If the relations between organisms 

 and the external world repaained constantly the same, no change 

 in the forms of organisms in the phylogenetic series would take 

 place ; and, since living substance has the property of reproduction, 

 by heredity the descendants would always be exactly like the 

 ancestors. Since, however, the conditions upon the earth's surface, 

 as upon every physical body, are continually changing, and since 

 the form of living substance, like every physical body, is under 

 the influence of its surroundings, it must likewise continually 

 change by adapting itself to the new conditions. Thus, there are 

 the two opposing factors of heredity and adaptation, and the result 

 of the action of these is expressed in the phylogenetic changes of 

 form. 



B. ONTOGENETIC DEVELOPMENT 



The old myth of the metamorphoses of the multiform Proteus 

 never found a more beautiful realisation than in the developmental 

 history of the individual. Just as the organic world as a whole 

 has undergone an unbroken change of form in the course of innu- 

 merable centuries, so the single individual, especially the multicel- 

 lular animal, during its development into the adult organism passes 

 through in the briefest time a long series of manifold forms until it 

 becomes like or approximately like its parents. It does not';belong 

 to the task of general phj'siology to follow the cycle of development 

 of individual groups of organisms ; by the great growth of the 

 fundamental ideas of Darwin and Haeckel our knowledge of indi- 

 vidual or ontogenetic development has expanded into an indepen- 

 dent science, embryology, the great importance of which for the 

 understanding of the present organic world has been demonstrated 

 during the last few decades. To-day no biologist or physician, who 

 has not became a blind specialist, is unequipped with embryo- 

 logical knowledge. But, although the study of the more special 

 facts of the ontogenetic development of form must be left to the 

 embryologist as his well-earned right, physiology has to deal with 

 certain general and elementary vital phenomena, upon which the 

 development of the individual rests. These are the phenomena of 

 reproduction. 



As should be the case with all vital processes, these phenomena 

 should be studied in the cell. The success of this method of treat- 

 ment has already l3oen demonstrated with reproductive phenomena ; 

 morphology has laboured here intelligently and has illumined the 

 whole field solely by means of cellular methods. As a result, we 

 are now oriented as to the minute details of the visible events. 



