54 ON THE EDUCATIONAL VALUE ii 



the evidence that the blood is propeHed hy the 

 heart tliroiioh the arteries, and returns hy tlie 

 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 pha^nomena 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, hut 

 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 



