UNCERTAINTIES IN INHERITANCE 295 



Various specialists on mental disorders have found reason to 

 believe in hereditary transmission in from 25 to 85 per cent, of 

 their patients, the diversity being doubtless in part due to the 

 great variety of nervous diseases. 



4. Many Uncertainties in Inheritance. — It is seldom possible 

 to say that a predisposition to a disease expressed in a parent 

 must be transmitted to the offspring. A predisposition to a 

 disease is rarely a sharp and definite character, such as we are 

 familiar with in " varieties " (of a species), which so frequently 

 breed true. It often means simply a slight disturbance of what 

 we may call the " gearing " of an organism — a slight derange- 

 ment of the normal sequences of the metabolism. It is an 

 unstable fluctuating variation. It is usually admitted that there 

 are families hereditarily predestined to be gouty or rheumatic, 

 but is any expert on either disease willing to stake his reputation 

 on the prediction that a particular gouty father is sure to have 

 gouty children ? 



It is not difficult to understand why individual prediction as 

 to the inheritance of predispositions to certain diseases is im- 

 possible. An individual inheritance is a mosaic of parental and 

 ancestral contributions. The reduction of these in the process 

 of maturation, the possibilities of fresh permutations and com- 

 binations in amphimixis, the variability of the germ-plasm under 

 the influence of nutritive oscillations in the blood-stream, the 

 probable occurrence of some sort of intra-germinal struggle 

 among the hereditary items (all living and self-assertive), the 

 importance of nurture in favouring the expression of one char- 

 acter and hindering that of another — the whole circumstances 

 of the case, in short, are so complex that prediction for individuals 

 is out of the question, no matter how certain we may be as to the 

 average result in 1,000 cases. A predisposition to a disease has 

 to run the gauntlet just like a predisposition to mathematical 

 insight or musical talent. There is just this difference, that 

 predispositions to disease are commoner, that they often occur 



