278 GENERAL CONSIDERATIONS [Cn. IX 



essential to its production? There is in biology no question 

 more important than this, and the answer is not so certain as 

 it ought to be. The fact that rigor occurs at a point at which 

 recovery of movement is still possible is not sufficient evidence 

 that metabolism has not ceased with the motion ; for I think it 

 has not been shown that with rigor " latent life " does not come 

 in. On the other hand, the fact that some bacteria are motion- 

 less in the absence of light (which can hardly be essential to 

 metabolism) would seem to indicate that conditions other than 

 those of metabolism are necessary to movement. This single 

 fact cannot, however, lead us to a definite answer, and our 

 inquiry, whether or not " stimuli " are essential to protoplasmic 

 movement, must still be regarded as unanswered. 



4. THE DETERMINATION OF THE DIRECTION OP 



LOCOMOTION 



As we watch an animalcule swimming across the field of view, 

 or as we see a larger organism moving, perhaps in a broken 

 line, towards any point, we think of its movements as controlled 

 from inside. Yet it is clear that if an organism is moving 

 definitely towards a point, it must be on account of some influ- 

 ence emanating from that point and falling upon the organism. 

 Without external directive influences of some sort there can be 

 no directed movements. 



This conclusion is confirmed by experiment. I have put an 

 amoeba into the apparatus already described (p. 186), so that 

 the chemical conditions of its environment were uniform ; con- 

 tact and temperature were also similar on all sides ; the direc- 

 tive action of gravity was annulled and all light was cut off. 

 At intervals the position of the amoeba was platted by the aid 

 of light reflected momentarily from below the stage of the 

 microscope and by means of a camera. Thus the path of the 

 amoeba was traced. A typical tracing made in this way is 

 reproduced in Fig. 72. Compare the devious path made under 

 these conditions with the straight path taken in response to 

 light (Fig. 53). The curious spiral twists and the turning of 

 the line upon itself are characteristic of all the tracings which 

 I have made under these conditions. Important also is the 



