of possible causes has to be discussed. Then, supposing the question, notwithstanding all these difficulties, to be so far settled in favour of the present system--there still arises that whole other series of difficulties with regard to the possibility of the spread of _other_ diseases by the practice, and with regard to the _extent_ of such spread, before we can arrive at any finale. This series of questions is almost as complex as the other; and it includes that great element of uncertainty--the question what interval of time may elapse between inoculation with a disease and its actual appearance. For if in several cases children break out with erysipelas immediately after vaccination, of course there is a certain presumption that vaccination has been the cause; but if the erysipelas only appears some years after, its connection with the operation may, though real, be impossible to trace. The matter standing thus, it seems to us almost a mystery how it was that the medical authorities of the early days of Jennerism were so cocksure of their conclusions--until we remember that in arriving at those conclusions they practically _ignored_ all these other points that I have mentioned, like changes of Sanitation, spontaneous decline of Small-pox, the spread of other diseases, etc., and simply limited themselves to one small aspect of the problem. But now, after this interval of time, when the neglected facts and aspects have meanwhile _forced_ themselves on our attention, how remarkable is the change of attitude as evidenced by the finding of the late Royal Commission! (1896). From all this do not understand me to deride Science--for I have no intention of doing that; on the contrary, I think the debt we owe to modern investigation quite incalculable; but I only wish to warn you how complex all these problems are, how impossible that notion of settling even one of them by a cut-and-dried intellectual formula. But you will ask (for this is the second point I mentioned some little time back) _how_ people's emotions and feelings come in to colour their scientific conclusions? And the answer is--very simply, namely by directing their choice as to what aspects of the problem they will ignore and what aspects they will envisage; by determining their point of view, in fact. To return to that illustration of several portrait-painters painting the same face; just as each painter is led by his feeling, his sympathies, his general temperament, to select certain points in the face and to pass over others, so each group of scientific men in each generation is led by its sympathies, its idiosyncrasies, to envisage certain aspects of the problems of the day and to ignore others.