In the investigation of microscopic organisms--the 'infinitely little,' as Pouchet loved to call them--and their doings in this our world, M. Pasteur has found his true vocation. In this broad field it has been his good fortune to alight upon a crowd of connected problems of the highest public and scientific interest, ripe for solution, and requiring for their successful treatment the precise culture and capacities which he has brought to bear upon them. He may regret his abandonment of molecular physics; he may look fondly back upon the hopes with which his researches on the tartrates and paratartrates inspired him; he may think that great things awaited him had he continued to labour in this line. I do not doubt it. But this does not shake my conviction that he yielded to the natural affinities of his intellect, that he obeyed its truest impulses, and reaped its richest rewards, in pursuing the line that he has chosen, and in which his labours have rendered him one of the most conspicuous scientific figures of this age. With regard to the earliest labours of M. Pasteur, a few remarks supplementary to those of M. Radot may be introduced here. The days when angels whispered into the hearkening human ear, secrets which had no root in man's previous knowledge or experience, are gone for ever. The only revelation--and surely it deserves the name--now open to the wise arises from 'intending the mind' on acquired knowledge. When, therefore, M. Radot, following M. Pasteur, speaks with such emphasis about 'preconceived ideas,' he does not mean ideas without antecedents. Preconceived ideas, if out of deference to M. Pasteur the term be admitted, are the vintage of garnered facts. We in England should rather call them inductions, which, as M. Pasteur truly says, inspire the mind, and shape its course, in the subsequent work of deduction and verification. At the time when M. Pasteur undertook his investigation of the diseases of silkworms, which led to such admirable results, he had never seen a silkworm; but, so far from this being considered a disqualification, M. Dumas regarded his freedom from preconceived ideas a positive advantage. His first care was to make himself acquainted with what others had done. To their observations he added his own, and then, surveying all, came to the conclusion that the origin of the disease was to be sought, not in the worms, not in the eggs, but in the moths which laid the eggs. I am not sure that this conclusion is happily described as 'a preconceived idea.' Every whipster may have his preconceived ideas; but the divine power, so largely shared by M. Pasteur, of distilling from facts their essences--of extracting from them the principles from which they flow--is given only to a few. * * * * *