23 THE EVOLUTION THEORY 



an egg in it, but she does so even if the egg be taken out beforehand, 

 or if a hole be made in the bottom of the cell, so that the honey which 

 is to serve the larva for food when it emerges from the egg runs 

 out (Fabre). Her instinct is calculated for filling the cell once with 

 honey, and once laying an egg in it, because such disturbances as we 

 may cause artificially do not occur or occur very rarely in natural 

 conditions. There are countless facts of this kind, for every instinct 

 and every adaptation can, in certain circumstances, go astray and 

 become inappropriate. This should be considered by those who still 

 persist in opposing the theory of selection, for herein lies one of the 

 most convincing proofs of its correctness. Adaptations can only arise 

 in reference to the majority of occurrences, and variations which are 

 only useful in an individual case must, according to the principle, 

 disappear again. Adaptation always means the establishment of what 

 is appropriate in an average number of cases. 



Therefore the inappropriate reaction of the margin of the iris to 

 an artificial double stimulus affords additional reason for regarding 

 regeneration as an adaptive phenomenon. If it were the outcome of 

 an adaptive force it could never be inappropriate ; and if it were the 

 operation of a general and primary power of the organism it would be 

 exhibited by the nearly -related frog as well as by the newt. But, in 

 the frog, extraction of the lens gives rise only to a sac-like prolifera- 

 tion of the cells of the iris margin, which form no transparent lens, 

 but an opaque cluster of cells, which destroys vision altogether. It 

 appears, therefore, that the frog no longer requires the power its 

 ancestors possessed of regenerating a lost lens. 



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