42 GENETICS AND EUGENICS 



sitates a particular response when certain external stimuli are 

 operative, not at all because such a response has before been 

 made by the ancestors. Having denied that action of the 

 individual can affect the germ-plasm within it, they can con- 

 ceive of no mechanism for the transmission of habits formed 

 by the individual, and so deny the existence of such trans- 

 mission. 



On the neo-Lamarckian view a hen sits on eggs because 

 her ancestors have formed the habit of incubating eggs; on 

 the Weismannian view the hen sits on eggs because she can- 

 not help doing it; when she is in a certain physiological state 

 and the nest of eggs is there, she sits, and that is all there is 

 to it. Neither of these views is very satisfying. On one 

 hand the neo-Lamarckian fails to explain how the first hen 

 came to incubate, which the Weismannian glibly states is 

 just because she is built that way; her germ-plasm necessi- 

 tates it. On the other hand, the Weismannian can give us 

 no suggestion as to how structural conditions of the germ- 

 plasm can cause a hen to sit rather than to crow, when a nest 

 of eggs is before her, but the well-established effects of 

 internal secretions come here to his rescue. 



The whole question of the relation of instincts to inheri- 

 tance is very perplexing. At present we can make very little 

 out of it, yet there can be no doubt that it concerns vitally 

 our fundamental theories of evolution and such applied fields 

 as Eugenics. 



The correct attitude in the study of instincts is maintained 

 by those who are seeking to learn how much each instinct 

 involves, and to what extent imitation and education supple- 

 ment or modify it. So far as possible each instinct should be 

 resolved into terms of response to external chemical or physi- 

 cal changes, or to internal physiological states. For example 

 it was observed many years ago that certain small Crustacea 

 instinctively swim toward a light. More careful study showed 

 that they do so only under particular conditions. If the 

 temperature of the water is raised, or its salinity increased, 

 the animal may reverse its response and swim away from the 



