3] 



GEOTAXIS 



123 



P 



the upper side and then travel obliquely, not vertically, along 

 it toward the upper part of the tube, thus into strata of con- 

 stantly diminishing pressure. If weight controlled in any way 

 their movements, they should move vertically as from 1 to 2 

 (Fig. 27) until they meet the side of the glass. Then they 

 should move off, as to 3, then vertically to 5, and so on. Since 

 they do not so move, gravity, JENSEN thinks, cannot be said to 

 act directly. In criticism of this conclusion it may be urged 

 that it is without proper foundation, for 

 if an organism whose irritability (in- 

 stincts) would lead it to move verti- 

 cally is mechanically unable to do so 

 exactly, it will do so as far as practi- 

 cable. This observation cannot, there- 

 fore, be said to militate against the 

 second theory. Finally, there is this 

 positive objection to JENSEN'S theory 

 that it is applicable only to geotaxis in 

 water animals, and can therefore be 

 only a special explanation of geotaxis. 

 On the other hand, there is evidence 

 which is opposed to the first theory and 

 favors directly the second. And JEN- 

 SEN has himself contributed some of 

 this evidence. He put Urostyla into 

 a glass tube containing a 0.5% aque- 

 ous gelatine solution. They showed 

 no tendency to go upwards. At the 

 expiration of 20 hours many deaths 

 had occurred, but some normally ac- 

 tive individuals were still at the lower end of the tube. Why 

 this loss of geotaxis ? JENSEN believes it due to the fact that 

 the difference in pressure of the successive layers did not 

 increase proportionally to the increase in resistance of the 

 solution. I would suggest that it may be due to the fact 

 that the weight of the body of the Protista is now relatively 

 less than that of the solution, so that the organism, tending 

 to move against resistance, comes to lie at the bottom of the 

 buoyant fluid, hence appears positively geotactic. 



FIG. 27. Hypothetical line 

 of migration of Parame- 

 cium in an inclined tube, 

 upon the assumption that 

 gravity acts directly to 

 determine direction of 

 locomotion, according to 

 the conception of JEXSEN*. 

 The arrow at p indicates 

 the direction of the pull 

 of gravity; 1, 2, 3, 4, 5, 

 successive positions occu- 

 pied by the Paraineciuni. 

 (From JEXSEX, '93.) 



