508 HUMAN BIOLOGY 



forms of insanity equally well characterized are recognized 

 and some are probably inheritable. 



A great mass of suspected and uncertain material is 

 presented for consideration in this field which has usually, 

 and doubtless some times properly, been explained by 

 assuming that what is inherited is not any specific disease 

 but a general instability of the nervous system on the basis 

 of which variously classifiable disorders and diseases are 

 developed. This is the kind of assumption which has in the 

 past frequently been made for other conditions and has as 

 often been replaced with advancing knowledge. 



LONGEVITY 



It has been increasingly recognized of late that the length 

 of life of the individual is a measurable biological phenome- 

 non, the analysis of which might uncover very interesting 

 facts. It is, of course, a common impression that length of 

 life is determined in considerable measure by inheritance. 

 Some families are thought to be notably long lived. That 

 the condition is counter-balaqced by equally well-marked 

 short hved families is possible but this is in the nature of the 

 case less easy to be sure about. When an individual lives a 

 long time we think naturally of his constitution as a respon- 

 sible factor and when his ancestry and immediate relatives 

 also survive, the constitutional factor becomes more and 

 more apparent. But when an individual dies young, the 

 disease of which he died or the accident of fate which carried 

 him off is the impressive feature. Suffice to say that observa- 

 tions on selected families of animals, fruit flies and guinea 

 pigs particularly, have shown that length of life whether 

 short or long is a definite family characteristic and have 

 given us some clues regarding its hereditary transmission. 



It has not been sufficiently recognized that this matter is 

 definitely related to the broad question of the inheritance of 

 disease. To make it plain that there must be such an intimate 

 relationship it is only necessary to point out that when the 

 individual dies it is most usually from some definite and 

 immediate disease. There are, it is true, some instances, and 

 these in the long lived exclusively, where death comes in 

 such a way that the rational description of it is comprised in 



