CHAPTER XIII 
SUMMARY AND GENERAL CONCLUSIONS 
Tue 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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