THE CYCLE OF LIFE 373 



reversal of the process. There is a quick or slow descent 

 to the vita minima of senescence, ending in natural death, 

 if violent death has not previously intervened. Varied 

 as the hfe-histories are, there is always the same general 

 phenomenon of cychcal development. 



The shape of the curve differs greatly in different types. 

 Some have a short youth, e.g. Aphides, which are ahnost 

 hke adults at birth and set to work at once ; while others 

 have a long youth, e.g. frogs, which spend about three 

 months in the larval state. Some have a prolonged 

 maturity, e.g. most birds and mammals ; while others have 

 it soon cut short, e.g. May-flies, which are sometimes 

 Hterally insects of a day. There may be prolonged adoles- 

 cence, as in an eel, or a precocious maturity, as in a rat. 

 Two general ideas should be borne in mind (a) that life- 

 histories differ from one another in the lengthening out 

 or shortening down (sometimes even telescoping) of parti- 

 cular periods ; and (6) that they differ much more inti- 

 mately in the details of the curve — ^in the minor ups and 

 downs — ^which mark the vicissitudes of days and seasons, 

 and the often correlated internal periodicities. 



The Continuance of Life. — ^A chronometer weU-woimd 

 can keep agoing for a long time, but it eventually comes 

 to a standstill, and so does the organism. An intricate 

 device like the famous Strassbuxg clock may go through 

 a compHcated performance, with processions of figures 

 and the like, but eventually the mechanism runs down 

 and the show is over. So is it with the organism. But 

 there are several big differences between an organism and 

 a mechanism, and one is that the organism normally 

 gives origin to other organisms hke itself or shares in so 

 doing. It multiphes or reproduces itself. 



