304 HEREDITY AND DISEASE 



in development. This nurture is to some extent in our hands. 

 Adaptive nurture, individual hygiene, change of habits and 

 habitat, and the like may prevent or inhibit the expression of a 

 predisposition. In short, the organism is extremely plastic — 

 for the better as well as for the worse. 



An organism with a predisposition to a constitutional disease 

 — let us say albuminuria, asthma, gout, diabetes, or some ner- 

 vous disorder — is obviously handicapped, more or less terribly 

 according to the strength of the predisposition. There is a 

 struggle for existence. But in this struggle a most momentous 

 factor is nurture in the widest sense — the conditions of function 

 and environment. If these favour the morbid elements in 

 the inheritance, the organism has to fight a battle with two 

 fronts, which is seldom hopeful. But if well-adapted conditions 

 of life be secured, and secured early, there is always considerable 

 hope that nurture may inhibit the full expression of the un- 

 desirable elements in the inherited nature. " He that will to 

 Cupar, maun to Cupar " is a familiar Scotch proverb, but the 

 other side of it is the " plasticity of the organism." 



(5) It seems to us that even expert writers have sometimes 

 exaggerated the necessity of the persistence of constitutional 

 taints and defects ; for as it is well known that a highly advan- 

 tageous variation may fail to persist, why may not this be 

 equally true of one that is highly disadvantageous ? A " retro- 

 grade variety " — that is, one which has lost one of the charac- 

 teristics of the parent species — may arise in our garden and 

 breed true. Why may not something analogous occur in a 

 peculiarly vulnerable stock ? Why may not the vulnerability, 

 the disadvantageous predisposition, disappear ? Apart from 

 natural selection, sexual selection, and the like, it may be that 

 the subtle process of germinal selection is sometimes able prac- 

 tically to eradicate an abnormal or morbid peculiarity. 



(6) Crosses between wheat-plants immune to rust and others 

 susceptible to rust yield hybrids which are all susceptible. But 



