54 ON THE EDUCATIONAL VALUE n 



the evidence that the blood is propelled by the 

 heart through the arteries, and returns by the veins 

 that, in short, the blood circulates. 



Suppose our experiments and observations have 

 been made on horses, then we group and ticket 

 them into a general proposition, thus : all horses 

 have a circulation of their blood. 



Henceforward a horse is a sort of indication or 

 label, telling us where we shall find a peculiar 

 series of phenomena called the circulation of the 

 blood. 



Here is our general proposition, then. 



How, and when, are we justified in making our 

 next step a deduction from it ? 



Suppose our physiologist, whose experience is 

 limited to horses, meets with a zebra for the first 

 time, will he suppose that this generalisation 

 holds good for zebras also ? 



That depends very much on his turn of mind. 

 But we will suppose him to be a bold man. He 

 will say, " The zebra is certainly not a horse, but 

 it is very like one, so like, that it must be the 

 ' ticket ' or mark of a blood-circulation also ; and, 

 I conclude that the zebra has a circulation." 



That is a deduction, a very fair deduction, but 

 by no means to be considered scientifically secure. 

 This last quality in fact can only be given by 

 verification that is, by making a zebra the subject 

 of all the experiments performed on the horse. Of 

 course, in the present case, the deduction would be. 



