As another illustration of my meaning, consider the Fixed Stars, so-called. These stars in their groups and clusters, which we know so well by sight, have remained apparently in the very same, or nearly the same, relative positions during all the 2,000 or 3,000 years that we have any record of the shapes of the Constellations. Yet now by minute telescopic and spectroscopic examination we know that they are moving, and have been moving all the time, in various differing directions with great velocities, amounting to miles per second. Nevertheless, so great are the spaces concerned, so great the times, that all this long period has not sufficed to bring them into any greatly changed attitude with regard to each other! What would you think of an intelligent foreigner who, coming to England to study the game of cricket, remained on the cricket field for a quarter of a minute--during which time the players would have hardly changed their positions--and having noted a few points, went away and wrote a volume on the laws of the game? And what are we to think of poor little Man who, having noted the stars for a few centuries, is so sure that he understands their movements, and that he is versed in all the "ordinances of heaven." Thus it would appear that every Nature-problem is so enormously complex that it can only be got at by what we have called the Method of Ignorance. Let us take a practical Science problem like that of Vaccination. The question here, put in its simplest terms, seems to be, Whether Vaccination, with calf or human lymph, prevents or alleviates Smallpox; and if it does, whether it does so without engendering other evils at least as great. At first sight this may appear to you a very simple question, and easy to solve; but the moment you come to think about it, you see its extreme complexity. In the first place, it is obvious that in a question like this, individual cases afford no test. It is obvious that the fact that A. is vaccinated and has not taken small-pox proves nothing, for there is nothing to show that he would have taken it if he had not been vaccinated. And when you have got people vaccinated by the hundred and the thousand, you still are not certain; for these people may belong to a certain class, or a certain locality, or may have certain habits and conditions of life, which may account for their comparative immunity, and these causes must be eliminated before any definite conclusion can be reached. Thus it is not till the great mass of the population is vaccinated that we can expect reliable statistics. But the introduction of a practice of this kind on so great a scale necessarily takes a long period of years, and meanwhile changes are taking place in the habits of the people, Sanitation is being improved, customs of Diet are altering, possibly (as so often happens in the history of an epidemic) the disease, having run its course, is beginning spontaneously to decline. And thus another series