ABILITY OF ANIMALS TO KEEP TIME WITH RHYTHM 445 



movement which spreads to their neighbors, and gradually the 

 members of the whole colony are brought into synchronous 

 rhythmic movement. Peairs (9) reports that fall web-worm 

 larvae engage in a rhythmic swaying movement which is syn- 

 chronous in the whole colony ; and he argues that it is probable 

 that the rhythm is conveyed from larva to larva by slight move- 

 ments of the web on which all rest. McDermott (5) mentions 

 the same phenomenon in a web-worm (same species ?). In all 

 these cases one can conceive how the synchronism is attained, 

 without ascribing to the animals anything beyond their well- 

 known powers; for the " imitated " rhythm is conveyed mechan- 

 ically to the animal's body and to its muscles. 



Without such a mechanical transfer, it is a question whether 

 animals below man have ever been really observed to keep time 

 with an external rhythm. A number of naturalists, as quoted 

 below, claim to have observed such synchronous rhythms; but 

 none of their reports are sufficiently detailed and exact to 

 prove that their observations were free from illusion. There are 

 three fertile sources of illusion in this matter, as follows: 1. The 

 observer cannot give his attention equally to a number of objects 

 at the same moment. It is impossible to watch even two moving 

 objects and tell whether their movements are simultaneous, un- 

 less the objects are very conspicuous, or very close together, or 

 both. This seems to account for the fact, e.g., that a man writes 

 to me from California asserting that he has seen flocks of geese 

 and of cranes in flight all moving their wings synchronously. 

 Shull 1 makes a similar criticism of a supposed case of synchron- 

 ism. 2. We all are prone to subjective accentuation, to a sub- 

 jective rhythm, and to the illusion that this is an objective 

 rhythm. 3. Many of these observers report a " high degree ' 

 of synchronism. Now, unless the synchronism is perfect, unless 

 is includes all the animals under observation, the observer is 

 liable to a statistical fallacy. For example, where a large number 

 of fireflies are flashing at slightly different rates there must be 

 a great amount of accidental synchronism ; to determine whether 

 there is a degree of synchronism not due to the laws of chance, 

 one would need to make a statistical examination, unless the 

 fireflies are all in perfect synchronism. 



1 Shull, A. F. The Stridulation of the Snowy Tree-cricket (Oecanthus niveus) 

 Canadian Entomologist, 1907, 39, 213-225. 



