MISUNDERSTANDINGS: IRRELEVANT CASES 183 



provoking stimulus and the expression of the innate tendency or 

 predisposition is more or less arbitrary various kinds of stimuli 

 will have the same result ; whereas the relation between an environ- 

 mental influence and the induced modification is more or less 

 constant similar influences having similar results and is more 

 strictly causal. An external stimulus may provoke the expression 

 of a germinal variation, as when a mouse provokes hysteria; but 

 this is different physiologically from what occurs when the sun 

 produces sun-burning. 



A certain abnormal psychosis, which may not have been hinted 

 at during early years, suddenly emerges under provocation. It is 

 carelessly spoken of (even in the law courts) as due to that 

 provocation a fright, a wound, a debauch, a railway accident, 

 a night's exposure, and so on, and it is carelessly thought of as 

 " acquired " ; it is recovered from, but it re-appears in the off- 

 spring : therefore an acquired character may be transmitted. But 

 there is the strongest probability that what was called an acquired 

 psychosis was primarily germinal, and might have emerged under 

 quite different stimulation for instance, under the normal events 

 of puberty and parturition. 



Another version of this misunderstanding is seen in references 

 to the improvement of a breed in the course of generations, as the 

 result, it is supposed, of functional modifications. Practice makes 

 perfect in the individual, therefore also in the race. But we have 

 seen no cases cited where the results were not hopelessly complicated 

 by the occurrence of selection and elimination, which, by acting on 

 constitutional variations, may quite well account for what is hastily 

 referred to modification-inheritance. 



Herbert Spencer was keenly aware of the misunderstanding which 

 we have been discussing. " Such specialities of structure as are 

 due to specialities of function are usually entangled with specialities 

 which are, or may be, due to selection, natural or artificial. In 

 most cases it is impossible to say that a structural peculiarity which 

 seems to have arisen in offspring from a functional peculiarity in 

 a parent is wholly independent of some congenital peculiarity of 

 structure in the parent, whence this functional peculiarity arose. 

 We are restricted to cases with which natural or artificial selection 

 can have had nothing to do, and such cases are difficult to find." 



Yet it is strange that he should point to such facts as the following : 

 the bones of the wing in the domestic duck weigh less and the bones 



