"In this undertaking to fight for those who were unfairly dealt with, he displayed in another direction that very conspicuous trait which, as displayed in his Alpine feats, has made him to many persons chiefly known: I mean courage, passing very often into daring. And here let me, in closing this little sketch, indicate certain mischiefs which this trait brought upon him. Courage grows by success. The demonstrated ability to deal with dangers produces readiness to meet more dangers, and is self-justifying where the muscular power and the nerve habitually prove adequate. But the resulting habit of mind is apt to influence conduct in other spheres, where muscular power and nerve are of no avail--is apt to cause the daring of dangers which are not to be met by strength of limb or by skill. Nature as externally presented by precipice ice-slopes and crevasses may be dared by one who is adequately endowed; but Nature, as internally represented in the form of physical constitution, may not be thus dared with impunity. Prompted by high motives, John Tyndall tended too much to disregard the protests of his body. "Over-application in Germany caused absolute sleeplessness, at one time, I think he told me, for more than a week; and this, with kindred transgressions, brought on that insomnia by which his after-life was troubled, and by which his power for work was diminished; for, as I have heard him say, a sound night's sleep was followed by a marked exaltation of faculty. "And then, in later life, came the daring which, by its results, brought his active career to a close. He conscientiously desired to fulfil an engagement to lecture at the British Institution, and was not deterred by fear of consequences. "He gave the lecture, notwithstanding the protest which for days before his system had been making. The result was a serious illness, threatening, as he thought at one time, a fatal result; and notwithstanding a year's furlough for the recovery of health, he was eventually obliged to resign his position. But for this defiance of Nature, there might have been many more years of scientific exploration, pleasurable to himself and beneficial to others; and he might have escaped that invalid life which for a long time he had to bear. In his case, however, the penalties of invalid life had great mitigations--mitigations such as fall to the lot of few.