CHAPTER XIII 



SUMMARY AND GENERAL CONCLUSIONS 



The question of the origin of the adaptations with which 

 the last three chapters have so largely dealt is one of the 

 most difficult problems in the whole range of biology, and 

 yet it is one whose immense interest has tempted philoso- 

 phers in the past, and will no doubt continue to excite the 

 imagination of biologists for many years to come. No pre- 

 tence has been made in the preceding pages to account for 

 the cause of a single useful variation. We have examined 

 the evidence, and from this we believe the assumption justi- 

 fied that such variations do sometimes appear. The more 

 fundamental question as to the origin of these variations has 

 not been taken up, except in those cases in which the adap- 

 tive response appeared directly in connection with a known 

 external cause. But these kinds of responses do not appear 

 to have been the source of the other adaptations of the 

 organic world. Our discussion has been largely confined to 

 the problem of the widespread occurrence of adaptation in 

 living things, and to the most probable kinds of known 

 variations that could have given rise to these adaptations. 

 But, to repeat, we have made no attempt to account for the 

 causes or the origin of the different kinds of variation. 



Nageli, in speaking of the methods of the earlier theorists 

 in Germany, remarks with much acumen : " We might have 

 expected that after the period of the Nature-philosophizers, 

 which in Germany crippled the best forces that might have 

 been used for the advance of the science, we should have 

 learnt something from experience, and have carefully guarded 



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