GENERAL SUMMARY AND CONCLUDING REMARKS 



to some external influence, or to internal variations of the 

 nervous system of definite or indefinite origin, we cannot say. 

 On more than one occasion, I have appealed to natural 

 selection as a probable explanation of the method by which 

 some particular piece of adaptation has been built up. One of 

 the criticisms to which this theory is sometimes subjected is 

 that we ought to be able to observe it in operation providing' 

 it were of such universal application. Much depends upon 

 how much we expect to see. We cannot of course hope r 

 within the short space of a human lifetime, to witness the 

 completion of a process, to see some great change accomplished 

 by virtue of some specific inherited disposition ; but because we 

 cannot be present while the whole wall is being built we are 

 not thereby debarred from seeing some of the bricks laid. 

 Wherever we find failure — failure to escape the dangers 

 attendant on migration, failure to secure food under normal 

 conditions, failure to attain the goal of reproduction — there 

 we have elimination with the corresponding brick laid in the 

 development of some characteristic which is of survival value. 

 Take as an example the nesting instinct of the Eeed Warbler. 

 These birds weave their nests to three, four, five, six or seven 

 reeds ; two would clearly be insufficient to hold the nest ; 

 three do not always afford the necessary security, the majority 

 are therefore woven to more. We can observe that a nest 

 woven to three reeds is liable to bring disaster to the offspring, 

 though perhaps not often, and we can infer that if it were 

 woven to two that danger would be increased. Now it is 

 more simple to find two reeds conveniently placed than to find 

 three, more simple to find three than four. If then the 

 number of reeds made use of were a matter of small moment, 

 why should the number of nests woven to more than three be 

 in so large a preponderance? What determines individual 

 behaviour in this respect? Does the same individual make 

 use of two reeds one year and fail, but five reeds the next and 

 succeed ? Or do some individuals always lay the foundations 

 of their nest more securely than others ? Since experience as 



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