RHYTHM IN NATURE — FLATTELY. 391 



periods or revolution time in which they pass through a cycle or 

 oscillation, the period varying from one type of cell to another. 



The contraction of the heart is an example of a vital rhythm which 

 can escape nobody. The series of movements taking place in the 

 heart during one complete beat constitutes what is known as a 

 "cardiac cycle." This, again, is essentially an age cycle, although 

 the rapid recovery after fatigue tends to obscure its real nature. 

 Disregarding the immediate causes underlying the repetition of the 

 cycle, the point to notice is that rhythm is associated with efficiency. 

 The rhythmic method represents the best means of accomplishing a 

 purpose, and may, in fact, be regarded as an evolutionary goal 

 toward which all life processes are tending. To take a concrete ex- 

 ample: It is certain that the circulation in vertebrates is both a more 

 orderly and a more efficient process than the flow of blood in an 

 animal, let us say, like an earthworm. The correctness of this view 

 of coupling rhythm with efficiency is supported by the phenomenon 

 of the disordered and arhythmical action of the heart and lungs in 

 disease. The loud " bourdon " of the engines at a power station 

 conveys a most distinctly rhythmic impression, and a practiced ear 

 can readily detect the smallest of functional troubles by the modifi- 

 cation of the normal rhythmic note. 



We now turn to cases of periodicity of the external type. 



It is well known that not a few diseases, particularly those like 

 malaria, produce markedly periodic symptoms. It is an interesting 

 fact, however, that not only does the malarial parasite itself show 

 regular development cycles within the organism, but the mosquito, 

 the carrier of the parasite, is markedly periodic in its habits. Some 

 interesting observations have been made on the flight of the mosquito 

 in the course of the construction of the Panama Canal. 5 Gatun, 

 about 7 miles south of Colon, is one of the largest settlements in 

 the Canal Zone. Between January and March, 1913, more mosquitoes 

 were found there than in any settlement since the beginning of work 

 on the canal. The weekly catch of Anopheles was from 7,000 to 

 20,000. Just to the south of Gatun is a lake which seemed a likely 

 breeding ground for mosquitoes, but it was soon found that they did 

 not have their origin here. To the west of Gatun, across a part of 

 the old French canal, there was some flat land into which sea water 

 and mud from the American canal were being pumped. This land 

 was so located that, to reach the settlement, adult mosquitoes would 

 have to fly from half a mile to a mile across, or at right angles to, 

 the stiff breezes which prevail at Gatun. It was eventually discovered 

 " that a regular purposive flight of mosquitoes toward the town took 



8 Le Prince, J. A., and Orenstein, A. J., Mosquito Control in Panama, London, Putnam, 

 1916. 



