78 THE NATURE AND ORIGIN OF LIFE 



condition without changing anything in those already exist- 

 ing I inoculate the animal with anthrax bacteridia and 

 I verify the struggle of the sheep against the bacteridia. 

 Suppose the sheep is cured ; no physiologist, clever as he 

 may be, unless he has at his disposal a culture of anthrax 

 bacteridia, will be able to find how the sheep that has been 

 cured differs from the sheep before it was diseased. No 

 direct chemical or physiological analysis of the modifica- 

 tions which have taken place in the sheep can be made in 

 the present state of science. Let the observer be as con- 

 scientious as you please, while limited to our present pro- 

 cesses of investigation he will have to assert that the sheep 

 has not changed. And yet it has undergone a profound 

 transformation, but it is with relation to the anthrax bac- 

 teridia. It has become refractory to the disease called 

 anthrax ; a new inoculation of the malady will not bring 

 back the disease. We say it has acquired immunity with 

 relation to anthrax bacteridia. 



This example enables us to explain clearly the method 

 which we are led to use. Into the exterior conditions B 

 of the life of individual A we introduce a factor b ; it is this 

 factor b which will henceforth have to serve us as a reagent 

 in studying variations which its own influence determines 

 in A. In this way we reach a simple law by a natural 

 method, whereas if we had wished to analyse the variations 

 of A without the help of 6, we should run up against difficul- 

 ties equivalent to impossibility. 



This natural method of analysing facts has not yet, I 

 believe, been proposed as a general method of investigation ; 

 but it has already been applied, and fruitfully, in the field 

 of the physico-chemical sciences. It has led up in particu- 

 lar to the Law of Lenz : 



" The displacement of an electric current in the neighbour- 



