124 Causes and Course of Organic Evolution 



knowledge of structural and experimental details is still of 

 the most meager nature for animals. 



A fourth sense is the gyrotactic or rotatory, regarding which 

 we still know far too little. It was one of Darwin's rich con- 

 tributions to biology, when he showed that the organs of many 

 and great plant groups showed a gyrotactic or circumnutating 

 movement. It would be impossible here even to touch briefly 

 on all the views that have been expressed in explanation of the 

 phenomena. But when we find such movements exhibited in 

 pronounced manner by many motile bacteria, by swarmspores 

 generally, by the spermatozoids of plants and animals, by 

 Spirogyra filaments, as well as other rather simple but multi- 

 cellular algae, and increasingly developed as we pass to the 

 highest plants, this is partial proof of its far-reaching character. 

 That the phenomenon is mainly shown by living elongating cells, 

 that it proceeds in root-tips buried in the soil, as well as in 

 stem-apices exposed to light, that some flowering plants such 

 as the "stem twmers" practically owe their capacity for sur- 

 vival to its activity, are proofs of its relation to the living cell- 

 substance, and its tendency to be located in active free cells, 

 or in terminal growing areas of an organism. 



But we are ignorant as yet of any definite structure or plastid 

 of the cell that may govern or aid rotatory or circumnutation 

 movements. All are agreed, however, that such movements 

 are only effected in living and even young tissues, and are due 

 to varying turgidity of cells, and ultimately to physico-chemical 

 changes proceeding in the protoplasm. 



Direct continuity however of such sense-movements from 

 plants to animals has not, so far as the writer is aware, been 

 traced. Yet it is now experimentally demonstrated that "the 

 sense of rotation" is typical for vertebrates. The writer also sug- 

 gests the existence (chap. 15) of rotatory sense organs in nemer- 

 teans. For in these a portion of the auditory organ that seems 

 exactly to correspond to the organ of rotation in lower verte- 

 brates is present. The question therefore presents itself as 

 to whether such a sense and such a sense-movement exist 

 in simpler and specially in unicellular animals. The observa- 



