82 AMEBOID MOVEMENT 



direction than any other, and the fact of its greater simplicity, 

 doubtless are responsible for its wider acceptance. The recent 

 criticism to which this theory has been subjected, however, in- 

 dicates clearly enough that this theory does not really give a 

 very adequate idea of the processes involved in ameboid move- 

 ment after all, and in so far as feeding processes are concerned, 

 the theory does not seem to apply at all according to Schaeffer 

 ('16, '17). But it could hardly have been anything more than 

 excellent guesswork if the surface tension theory as advanced 

 by a number of writers had been found adequate, for the obser- 

 vational basis was very narrow, as the preceding pages have 

 shown, and as the succeeding pages further show. 



Not anything like a complete historical account of this theory 

 with its numerous modifications will be attempted here. It would 

 be a large undertaking, for nearly every biologist and biochemist 

 has expressed himself on the subject. It does not appear that 

 much is gained by merely recording the opinions, even of biolo- 

 gists, unless they are based on experimental or observational data, 

 preferably their own. Scientific questions are not decided by 

 ballot vote, and it is not apparent what value such a record 

 of opinions would have except the doubtful one of showing 

 whether the persons involved declared for or against the surface 

 tension theory. Moreover such an account would not be inter- 

 esting reading for those who want to know first of all what 

 amebas can do. Only the more important modifications of the 

 surface tension theory are applied to ameboid movements will 

 therefore be discussed and these modifications will be considered 

 important in proportion to the amount of observation or experi- 

 ment on which they are based. 



Attention has already been called in Chapter II to Berthold's 

 ('86) theory of ameboid movement, which was the first attempt 

 to explain this phenomenon on physical principles. As will be 

 remembered, Berthold thought that the nature of the ameba's 

 immediate environment determined when and in what direction 

 it should move, the source of the energy of movement being 

 supposed to be a decrease in the tension of the surface film of 

 the ameba, brought about by some factor in the ameba's imme- 

 diate environment. 



