3G2 W. von BUDDENBROCK 



sions like adaptation and natural selection are demonstrably 

 useless and where the only concern is to make clear the mechan- 

 ism of the occurrence," (1913, p. 452). I find, however, another 

 place where he openly states : ' that species manifesting tro- 

 pisms which would be incompatible with the reproduction or 

 maintenance of their kind must die out," which is in principle 

 an affirmation of the idea of selection. 



Whatever Loeb's position toward Darwin may be, it is cer- 

 tain, whether he admits it or not, that he needs' Darwin as an 

 ally, if he intends to explain the purposeful nature of tropisms. 



It is all the same to us whether harmful tropisms, the existence 

 of which we can rightfully contest, occur together with indifferent 

 and useful ones, if only the origin of the useful tropisms can be 

 established. 



Some examples will now be given of tropisms which serve 

 some purpose for the whole organism. 



There are many animals in which the tropism takes the form 

 of a movement of flight to avoid an enemy's attack. Certain 

 species of Cypris living in open water are rendered positively 

 geotropic and negatively heliotropic by mechanical disturbances; 

 that is, they seek the deepest and darkest part of the water. 

 On the other hand there are inhabitants of dark localities, 

 which, when disturbed, respond with positive heliotropism, 

 thereby avoiding an animal which is burrowing in the mud. 



In other cases the tropism of food getting may be seen. Loeb 

 himself gives as an example the behavior of the young cater- 

 pillar of Porthesia, which, creeping out of its winter nest can 

 only find the young leaves at the tip with the aid of positive 

 heliotropism. 



One case should be mentioned where movements of this kind 

 occur in response to need of air, for example, the water scor- 

 pion, Nepa, after using up its air becomes negatively geotropic 

 and positively heliotropic, 15 and it thus rises to the surface of 

 the water where it can get a new supply of air. In the same 

 category probably is the positive heliotropism of the Daphnids, 

 described by Loeb, which they manifest when certain acids, 

 especially carbonic acid, are added to the water. This tropism 

 evidently aids the animal in seeking fresher water levels. 



15 Baunacke, W. Statische Sinnesorgane bei den Nepiden. Morphol. Jahrb., 

 Abl. AnaU, 1912, Bd. 34. 



