20 PHYSIOLOGICAL CLASSIFICATION 



The various tactics to which animals and plants 

 resort when driven by necessity or threatened by 

 danger, and the reactions which they exhibit 

 towards external stimuli such as heat, light, 

 drought, moisture, contact, proximity of food 

 and of hostile influences, have long engaged 

 the attention of biologists for the reason, amongst 

 others, that the differently constituted nervous 

 system of invertebrate animals and the lack of 

 this system in plants, in protozoa, and in sponges, 

 impart a special character to their vital reactions, 

 in contrast with the more or less intelligent 

 responses of craniate vertebrates. 1 All the 

 tendencies or "tropisms" to which protoplasm 

 is liable are to be observed, either positively or 

 negatively, in every organism, and frequently, 

 at different times, both positively and negatively 

 in the same species. 



One of the most commonly observed reactions 

 is that known as phototaxis or heliotropism, these 

 terms being, however, not quite synonymous 

 inasmuch as the reaction to sunlight (i.e., helio- 

 tropism in the strict sense) is not the same 



1 Cf. J. von Uexkiill, " Leitfaden in das Studium der experi- 

 mentellen Biologic der Wassertiere." Wiesbaden, 1905. 



M. Verworn, "Allgemeine Physiologic," 4th edit., Jena, 1903. 

 English translation " General Physiology " from the 2nd edit, by 

 F. S. Lee, London, 1899. 



H. S. Jennings, " Behaviour of the Lower Organisms." New 

 York, 1906. 



J. E. Duerden, " On the Habits and Reactions of Crabs bearing 

 Actinians in their Claws." P. Zool. Soc. London, ii. 1905 (1906). 



