THE LAW OF HEREDITY 86 



In their various forms they are believed to be mani- 

 festations of a single evil, namely, the imperfect 

 nutrition of the brain or nervous system.^ The 

 earliest form of that evil is probably the depression or 

 nervous relaxation produced by the worries of civilised 

 life. It may first manifest itself in the simple guise of 

 indigestion, sleeplessness, irritability, headaches, or 

 general " lowness." By dint of double heredity, that is 

 to say, by its occurrence in both husband and wife, the 

 evil is aggravated in the second generation, and so on, 

 the members of the affected family being finally either 

 rendered unfit to Uve, or cured of their malady by 

 a sufficient admixture of healthy blood. Upon how 

 minute a scale Nature works Darwin shows in his 

 description of the life-germ which is transmitted in 

 generation. This life-germ is quite invisible to the 

 naked eye. Yet, "we must believe," says Darwin, 

 " that it is crowded with characters proper to both 

 sexes, to both the right and the left side of the body, 

 and to a long line of male and female ancestors 

 separated by hundreds or even thousands of genera- 

 tions from the present time," and that "these 

 characters, like those written on paper in invisible 



^ Dejerine, L'H6r6dit4 dans les Maladies du SysUme Nerveux, 

 Palis, 1886. 



