194 GENERAL BIOLOGY 



on account of the discoveries in Mendelian inher- 

 itance, 1 a great many biologists believe that the 

 development of the structural characteristics of 

 animal and plant individuals is dependent upon the 

 presence of " something " in the germ-cell to which 

 the name determiner is given. This determiner is a 

 physical entity of some sort, however, and very 

 different from the vis essentialis of Wolff. 



Vitalism and Mechanism. The more exact be- 

 comes our knowledge of the processes of differentia- 

 tion and development, the more wonderful appears 

 the delicate adjustment of forces that brings about 

 the final result. The correspondence of time and 

 place in development is at present particularly 

 difficult to comprehend. Why, for instance, does 

 an organ, let us say a finger, develop at precisely the 

 time and place necessary to produce a symmetrical 

 whole ? Why does an organ develop in apparent 

 anticipation of a subsequent need? The wtalists 

 believe that no known laws of matter can account 

 for the adaptation of means to the end, which we 

 are constantly confronted with in the study of mor- 

 phogenesis, and that it is necessary to postulate a 

 non-mechanical principle or " vital force," to which 

 various names are given, which is a guiding and con- 

 trolling agency in directing the course, not only of 

 development, but of life processes in general. A 

 sculptor in modeling a statue must have a pretty 

 clear idea in his mind of just what he expects to 



1 See next chapter. 



