FRANDSEN. — REACTIONS OF LIMAX MAXIMUS. 197 



character of the slime secretion of the animal. If accurate measure- 

 ments were made of the two regions of the body, we might obtain ex- 

 actly the relative weights of these two factors. By means of a spring 

 balance, the effectiveness of the mucus in counteracting gravity could be 

 ascertained with a fair degree of accuracy. A large number of such 

 observations in connection with geotactic tests might, finally, enable us 

 to state precisely what combination of the two factors — weight of 

 regions and strength of mucus — would be necessary to make an 

 animal positively or negatively geotactic. I have made no such calcu- 

 lations, and it would perhaps not be worth the trouble. The suggestion 

 is instructive, however, as indicating the possibilities of predicting, with 

 a certain degree of exactness, a phenomenon which seems at first sight 

 to be entirely haphazard. Perhaps perfect mathematical exactness 

 would, however, never be possible in this case, for, as I shall show a 

 little later, other factors of importance probably enter in to modify 

 the results. However, these too are not out of the reach of precise 

 definition. 



Certain slugs are negatively geotactic because gravity pulls the pos- 

 terior region of the body down faster than it does the anterior region. 

 Since in all slugs the posterior region somewhat exceeds in length the 

 anterior, we should expect all animals to respond in the same way, pro- 

 vided gravity acted in only a mechanical way. But about the same 

 number of slugs go down as go up. Therefore, there must be some 

 other factor, such as an inherent tendency, impelling these positive 

 slugs to seek the earth. But if so, is it not probable that all slugs 

 have this inherent tendency to move towards the earth, the tendency 

 being obscured in the negative slug by the superior force of the me- 

 chanical difficulties to be overcome ? The fact that positive slugs, 

 when deficient in means of resisting the pull of gravity, — that is, when 

 dry, — assume a negative geotaxis, shows that the inherent tendency is 

 sometimes obscured. If this hypothesis is true, then we ought to be 

 able, by diminishing the force of gravity, or better, by increasing the 

 animal's powers of resisting the disproportionate pull on the posterior 

 region, to make the negative animals become positive. Similarly, if 

 this mechanical difficulty of adhesion is the cause of negative geotaxis, 

 we ought, by increasing it, to be able to compel positive animals to be- 

 come negative. The first end may be attained by substituting for the 

 glass plate a wooden one, which will presumably offer the animal a 

 better chance of adhesion. The second end may be reached by substi- 

 tuting for the glass plate one which has been coated with vaseline or 



