560 THE BEGINNINGS OF LIFE. 



persist. That is to say, whilst they are individually 

 short-lived, and are most prone to give birth, at dif- 

 ferent times, to other organisms of the most varied 

 nature, they are also exceedingly apt to recur, quite 

 independently, just as certain crystalline forms are apt 

 to recur, when crystallizable compounds separate from 

 the state of solution, at different periods. 



Ephemeromorpks and c Alternate Generation? 



The studies which have revealed some of the nu- 

 merous and complex relationships existing between 

 the multitudinous varieties of c Ephemeromorphs,' have 

 been of cardinal importance for the science of Bio- 

 logy. They have taught us, not only that living matter 

 is formable de novo, but that it possesses inherent ten- 

 dencies of such a nature as to make it prone to undergo 

 variations in constitution, directly leading to variation 

 in external formj that transitions are always easy in 

 these early stages from the simpler vegetal to the 

 more complex animal modes of growth, and vice 

 'versa , and that, in the main, when placed under 

 favourable conditions, the different forms tend to 



Lichens as Lecidea geographica probably date from almost fabulous 

 periods, and even small patches are often of considerable age. I have 

 myself watched individuals for twenty-five years, which are now much 

 in the same condition as they were when they first attracted my notice. 

 Plants which endure without injury such extremes of temperature, and 

 conditions of the hygrometer, would seem, a priori, to be likely to have 

 great powers of longevity.' ('Introd. to Crypt. Bot.,' p. 418.) 



