88 IOWA ACADEMY OF SCIENCES. 



existing structure. Such modified development is taking 

 place for a longer or shorter period, commonly at the end 

 of the general development of the individual, that is, as 

 the individual is reaching maturity. But this development 

 is, as has already been set forth, the result of the inter- 

 action of an internal factor (heredity) and an external 

 factor (environment); or in other words it is the reaction 

 of the organism to external forces. Now, some individual 

 organisms, as I have explained above, will react differently 

 from others; or if we take the case where the external 

 factor is a change in the environment necessitating 

 increased function of some organ, some will react more 

 favorably than others, and such variations occurring 

 especially when needed, as above shown, will present 

 many cases of a selective value even in the first genera- 

 tion. If this be so, natural selection will account for this 

 perpetuation. Let us take, as a concrete example, the 

 case of the long neck of the giraffe. Neo-Lamarckians 

 would say that the efforts of the animal to reach high 

 foliage causes the neck to grow longer, and this being 

 inherited generation after generation, finally results in the 

 present condition. 



Neo-Darwinians would say that among many ancient 

 giraffes some accidentally had longer necks, and these, 

 getting more food in famine time, survived and perpetu- 

 ated their kind; the selection of such chance variations 

 from generation to generation resulting in the present 

 condition. 



The view here put forth is that among many developing 

 giraffes, some, by chance, possessed such powers of growth 

 that they were able to react better than others to the 

 external factor, the stimulus of reaching up, and as a result 

 of this reaction their necks grew longer; then they came 

 under the operation of natural selection. 



Furthermore, I believe it can be shown that variations 

 of the kind I have been discussing, namely, those which 

 arise w4ien the environment is gradually changed, will be 

 progressively adaptive; that is, with each change in envi- 

 ronment, variations will not arise indiscriminately in all 



