ever, which preceded an adequate knowledge of biology in the form of medical and sanitary knowledge, resulted in a high mortality rate, which is always a heavy drain on society, and therefore a great impediment to progress. Not only was the ordinary death rate high, but occasionally plagues swept over large areas making fearful havoc in the population. Authorities state that the black death in the fourteenth century took half the population of England. These pestilences were spread by the increased travel and trade made possible by the very progress which had been achieved in the control of nature. This period of the predominance of the natural sciences may be called the great period of natural selection. A denser population made possible by the increasing control of nature was held in check by a high death rate, uncontrolled because of the lack of medical knowledge. But these unfortunate conditions resulting from the unequal advancement of knowledge developed influences which were destined finally to reduce the evils. Disease and death have always seemed great enough evils to cause men to try to avoid them in more or less rational ways; but in a concentrated population these evils are brought forcibly to men's attention especially when they come in the form of a disastrous pestilence. Furthermore, increased association, which comes from a larger and denser population, is the chief means of developing sympathy and of arousing the desire to alleviate the sufferings of others. Hence the increased sympathy for others, and the more vivid realization of the amount of suffering in existence, became incentives for an increased effort to lessen the evils of disease. Moreover, other altered conditions caused these efforts to take a scientific turn. Previously superstitious beliefs had hindered the progress of science. Plagues were considered a visitation of the divine wrath, disease was treated with charms or with appeals to the saints, and the growth of anatomical knowledge was hindered by religious superstitions which forbade contact with dead bodies. But the new knowledge of the material world gradually lessened the hold of these superstitions and prepared the way for the scientific observation of the course of disease and the study of anatomy by the laboratory method.
In describing the effects of the natural sciences and the influences which have caused the development of the biological sciences, I do not mean to imply that the progress of knowledge has been continuous and uninterrupted, so that one particular period of history may be pointed out as having the conditions favorable for the origin of biology. There are a number of periods in which the forces here mentioned have been at work in greater or less degree and have influenced biological science. Perhaps at no time have they been more in evidence than at present, when, for example, modern conditions have turned people's attention to the ravages of tuberculosis, and have increased efforts to overcome