The quick intelligence was always working, sometimes wearily, sometimes eagerly, but working, unless absolute physical prostration forbade. When he is too exhausted, he complains: ‘Facts compel me to conclude that my brain was never formed for much thinking.’[446] But if so, he certainly lived contrary to his nature. He tells us that he cannot resist forming hypotheses on every subject.[447] Sometimes he bewails the tendency, realizing its drawbacks and dangers. Sometimes he gives way to it, recognizing its charm: ‘It is delightful to have many points fermenting in one’s brain.’[448] Speculation is fascinating. Theory gives form and texture to the fleeting drift and confusion of fact. Yet even when one indulges with most enthusiasm, a touch of humor shows that the satisfaction must be tempered with a certain lack of entire confidence: ‘That is a splendid fact about the white moths; it warms one’s very blood to see a theory thus almost proved to be true.’[449] For the wonder and the interest of Darwin is, that, with such an eager and perpetual bent toward theorizing, he could keep the bent so fully under control. As Karl Pearson puts it, generally: ‘Hundreds of men have allowed their imagination to solve the universe, but the men who have contributed to our real understanding of natural phenomena have been those who were unstinted in their application of criticism to the product of their imaginations.’[450] Surely no man applied such criticism more carefully, more conscientiously, more constantly than Darwin. He analyzes his own position and sees the dangers of it: ‘Living so solitary as I do, one gets to think in a silly manner of one’s own work.’[451] He sees constantly how theory interferes and warps the judgment: ‘I have not a doubt that before many months are over I shall be longing for the most dishonest species as being more honest than the honestest theories.’[452] The possibility of error haunts him, torments him, and he knows well how apt his own speculative disposition is to mislead: ‘What you hint at generally is very, very true: that my work will be grievously hypothetical, and large parts by no means worthy of being called induction, my commonest error being probably induction from too few facts.’[453] As a consequence he was ever on his guard against being led astray. The tempting little demon of hypothesis might be luring round the corner: ‘It is as difficult not to form some opinion as it is to form a correct judgment.’[454] But whatever opinion was formed must be corrected, must be adjusted, must be tested, by the cold and rigid measure of fact. As he says himself, ‘I have steadily endeavored to keep my mind free so as to give up any hypothesis, however much beloved (and I cannot resist forming one on every subject), as soon as the facts are shown to be opposed to it.’[455] And one who knew him well and had studied him carefully says the same thing with equal emphasis: ‘His long experience had given him a kind