166 THE PRINCIPLES OF HEREDITY 



of the first theory. It supposes that particular external influ- 

 ences affect the germ- plasm in particular ways. 



281. We have insisted, perhaps with wearisome reitera- 

 tion, that the doctrine of Natural Selection is quite incom- 

 patible with the hypothesis that variations are caused by the 

 direct action of the environment. If the one is true the 

 other cannot be true. But the incompatibility has not been 

 sufficiently recognized by biological thinkers. Probably this 

 confusion of really opposed ideas would never have arisen if 

 students of evolution had studied disease as closely as they 

 have studied heredity, or, on the other hand, if students of 

 medicine had devoted as much attention to the facts of adapt- 

 ation as they have devoted to disease. The naturalist 

 and botanist notes the close adaptation of every species to its 

 place in nature, and perceives that the adaptation can have 

 been brought about only through Natural Selection, or, in 

 some cases, through the transmission of acquirements. As a 

 rule, observation forces him to reject the Lamarckian doctrine. 

 But to his mind Natural Selection generally presents itself as 

 the sudden elimination of the unfittest by violent death. The 

 antelope is slain by the lion, or escapes by the exercise of 

 faculties which develop with use and so increase the general 

 strength and vigour. The tree is uprooted by the storm, or 

 develops a tougher fibre and sends its roots deeper. Seeing 

 only one aspect of the problem he perceives nothing incom- 

 patible in the belief that adaptation is due to Natural Selec- 

 tion, and the belief that variations are caused by the direct 

 action of the environment on the germ-plasm. He forgets 

 that there are instances of adaptation other than against the 

 causes of violent death. The medical man, on the other hand, 

 studies the individual rather than the race ; and is preoccupied 

 more with disease and other causes of slow decay than with 

 the sudden death of healthy and vigorous individuals. Like- 

 wise perceiving only one aspect of the problem, he ignores 

 the facts of adaptation, and very naturally concludes that 

 causes which enfeeble the individual must tend to cause 

 deterioration of the race. But if the causes of the slow deterior- 

 ation of individuals be studied in conjunction with the facts 

 of adaptation, it becomes impossible to doubt that Natural 

 Selection must long ago near the beginnings of life have 

 reduced the influence of the environment on the hereditary 

 tendencies of the germ-plasm to a minimum. 



282. It is quite possible, indeed it is more than probable, 

 that even amongst the highest animals, individuals occur 

 whose germ-plasm is more or less easily altered by external 



