92 THE STRUGGLE FOR EXISTENCE 



However, in experiments of this type there exists a great number of 

 different factors not exactly controlled, and a considerable difficulty 

 for the study of the struggle for existence is presented by the continu- 

 ous and regular changes in the environment. It is often mentioned 

 that one species usually prepares the way for the coming of another 

 species. Recollecting what we have said in Chapter II it is easy to 

 see that in such a complicated environment it is quite impossible to 

 decide how far the supplanting of one species by another depends on 

 the varying conditions of the microcosm which oppress the first 

 species, and in what degree this is due to direct competition between 

 them. In this connection one of the main problems of our experi- 

 ments with Protozoa has been to eliminate the complicating influence 

 of numerous secondary factors, and to apply such a technique of 

 cultivation as would enable one to form a perfectly clear idea as to 

 the nature of the factor limiting growth. This could not be done at 

 once and the technique of our first experiments presented all the usual 

 defects. Only later, taking into account certain suggestions of Amer- 

 ican authors, we made use of a synthetic medium for cultivating the 

 Protozoa, and the result furnished exceedingly clear data to a detailed 

 description of which we will soon pass. 



(4) A new property of the infusorian population distinguishing it 

 from that of yeast cells is that the infusorian population constitutes a 

 secondary population living at the expense of bacteria which it de- 

 vours. Thus here appears an elementary food chain: bacteria — > 

 infusoria. In our initial experiments the standardization of the 

 conditions of cultivation was only a quite superficial one. Without 

 taking any precautions as to an exact control of the physicochemical 

 properties of the medium and the number of growing bacteria, we 

 prepared the nutritive medium in the following manner: to 100 c.c. 

 of tap water 0.5 gr. of oatmeal was added; the whole was boiled for 10 

 minutes, left to stand and then the upper liquid was carefully poured 

 off, diluted 1^ times by water, and sterilized in an autoclave. After 

 this an inoculation of Bacillus subtilis was made, and the medium was 

 put into the thermostat at 32° for seven days in order to obtain an 

 abundant growth of bacteria. Before using, the medium was diluted 

 twice by tap-water, and without any further sterilization was put 

 into test tubes. (This was the so-called "oaten medium without 

 sediment." The "oaten medium with sediment" mentioned in 

 Chapter VI differs in its not being diluted by water before using, and 



