4 2o SCIENCE PROGRESS 



this latter being convertible into mechanical energy, electrical 

 energy, heat energy, or chemical energy. Like all energy 

 transformers, living cells have their phasic periods or revolu- 

 tion 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 towards 

 which all life processes are tending. To take a concrete 

 example : 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 practised ear can readily detect 

 the smallest of functional troubles by the modification 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. 1 Gatun, about 

 seven miles south of Colon, is one of the largest settlements in 

 the Canal zone. Between January and March 191 3, more mos- 

 quitoes 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 



1 Le Prince, J. A. and Orenstein, A. J., Mosquito Control in Panama. (London : 

 Putnam, 1916.) 



