The supreme means for keeping theory on a basis of fact is of course unfailing, persistent, ever-varied experiment. In the preceding chapter we have seen observation lead naturally to experiment; but experiment is observation guided, directed, and illuminated by theory. And assuredly no scientist ever had the love and the habit of experiment more firmly fixed than Darwin. The explanation of phenomena was all very well in its place, essential, absorbing, but he would have agreed absolutely with Aristotle as to the proper ordering of the process: ‘After this we shall pass on to the discussion of causes. For to do this when the investigation of the details is complete, is the proper and natural method, and that whereby the subject and the premises of our argument will afterwards be rendered plain.’[462] The fascination of experiment in itself was endless and almost sufficing: ‘The love of experiment was very strong in him,’ says his son, ‘and I can remember the way he would say, “I shan’t be easy till I have tried it,” as if an outside force were driving him.’[463] There was a sense of adventure about it, of discovery, and he loved to try things that seemed almost fantastic and absurd: ‘If you knew some of the experiments (if they may be so-called) which I am trying, you would have a good right to sneer, for they are so _absurd_ even in _my_ opinion that I dare not tell you.’[464] And more concisely and vividly: ‘I am like a gambler and love a wild experiment.’[465] The best comment on which is the excellent remark of Professor Castle: ‘Most advances in practical affairs are made by those who have the courage to attempt what others _with good reason think unattainable_. When such attempts have succeeded, the world simply revises its classification of things attainable and unattainable, and makes a fresh start.’[466]