14 GERMINAL SELECTION. 



esses that do and can take place at the same time in 

 the same species. Yet all this is necessary if we wish 

 to follow out the precise details of a given case. 



But perhaps the most discouraging circumstance of 

 all is, that in scarcely a single actual instance in 

 nature can we assert whether an observed variation is 

 useful or not — z drawback that I distinctly pointed out 

 some time ago.^ Nor is there much hope of better- 

 ment in this respect, for think how impossible it would 

 be for us to observe all the individuals of a species in 

 all their acts of life, be their habitat ever so limited — 

 and to observe all this with a precision enabling us 

 to say that this or that variation possessed selective 

 value, that is, was a decisive factor in determining the 

 existence of the species. 



In many cases we can reach at least a probable in- 

 ference, and say, for example, that the great fecundity 

 of the frog is a property having selective value, basing 

 our inference on the observation that in spite of this 

 fertility the frogs of a given district do not increase. 



But even such inferences offer only a modicum of 

 certainty. For who can say precisely how large this 

 number is? Or whether it is on the increase or on 

 the decrease? And besides, the exact degree of the 

 fecundity of these anmials is far from being known. 

 Rigorously viewed, we can only say that great fe- 

 cundity must be advantageous to a much-persecuted 

 animal. 



And thus it is everywhere. Even in the most in- 

 dubitable cases of adaptation, as, for instance, in that 

 of the strik ing protective coloring of many butterflies, 



^Die Allmacht der Naiursuchtung. A Reply to Herbert 

 Spencer. Jena, 1893, p. 27 et seq. [Also in the Contemporary 

 Review for September, 1893.] 



