THE PROBLEM 5 



under the given conditions of environment? Clements has character- 

 ized this phenomenon in the following manner: "The beginning of 

 competition is due to reaction when the plants are so spaced that 

 the reaction of one affects the response of the other by limiting it. 

 The initial advantage thus gained is increased by cumulation, since 

 even a slight increase of the amount of energy or raw material is 

 followed by corresponding growth and this by a further gain in re- 

 sponse and reaction. A larger, deeper or more active root system 

 enables one plant to secure a larger amount of the chresard, and the 

 immediate reaction is to reduce the amount obtainable by the other. 

 The stem and leaves of the former grow in size and number, and thus 

 require more water, the roots respond by augmenting the absorbing 

 surface to supply the demand, and automatically reduce the water 

 content still further and with it the opportunity of a competitor. 

 At the same time the correlated growth of stems and leaves is produc- 

 ing a reaction on light by absorption, leaving less energy available 

 for the leaves of the competitor beneath it, while increasing the 

 amount of food for the further growth of absorbing roots, taller stems 

 and overshading leaves" (Clements, '29, p. 318). 



(7) It is not difficult to see that for the study of the elementary 

 processes of the struggle for existence in animals we need experiments 

 of another type. We are interested in the processes of destruction 

 and replacing of one species by another in the course of a great num- 

 ber of generations. We are consequently concerned here with the 

 problem of an experimental study of the growth of mixed populations, 

 depending on a very great number of manifold factors. In other 

 words we have to analyze the properties of the growing groups of 

 individuals as well as the interaction of these groups. Let us make 

 for this purpose an artificial microcosm, i.e., let us fill a test tube with 

 a nutritive medium and introduce into it several species of Protozoa 

 consuming the same food, or devouring each other. If we then make 

 numerous observations on the alteration in the number of individuals 

 of these species during a number of generations, and analyze the fac- 

 tors that directly control these alterations, we shall be able to form 

 an objective idea as to the course of the elementary processes of the 

 struggle for existence. In short, the struggle for existence among 

 animals is a problem of the relationships between the components in 

 mixed growing groups of individuals, and ought to be studied from the 

 viewpoint of the movement of these groups. 



