in VARIATION UNIVERSAL 147 



successive generations, to hasten or to retard in a 

 marked manner the nervous development. This may 

 be : at all events the facts are interesting, and show 

 that methods may be devised through which an in- 

 fluence may be exerted on constitutional peculiarities. 

 Such facts give us hopes of seeing where the cause of 

 physiological variability may be sought. 



To all that has been said concerning variability, 

 morphological and physiological, we must add that 

 this variability is not limited to any group of plants or 

 animals, to any time of life, nor to any geological epoch. 

 From the very beginning of life variation is apparent. 

 Take a number of eggs of the same frog, in conditions 

 apparently identical : some are early and some late in 

 developing into tadpoles. Tadpoles acquire their 

 limbs and lose their tail at very different intervals, 

 although all live in the same aquarium, are born from 

 the same parents, and enjoy the same food ; and if you 

 put some of them into the same toxic solution, some 

 are certain to die much earlier than the others, and if 

 the time during which they remain in the dangerous 

 medium is not too long, some certainly will recover, 

 while the others die. Of the whole progeny of the same 

 parents, whether two or two hundred young are pro- 

 duced at one time, no two are exactly alike : noticeable 

 differences exist, if not in the external, certainly at 



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