Every science has its technical vocabulary, consisting in part of terms coined for explicit purposes and in part of words borrowed from ordinary discourse and used with greater or lesser degrees of precision. Words of the latter sort pose curious problems, some of them familiar to those historians of science concerned with, for example, what Galileo meant by forza and Newton by attraction. Indeed, analogous problems face any historian seeking to understand the older meanings of terms still in use today.

Roger Bacon has often been victimized by his friends, who have exaggerated and distorted his place in the history of mathematics. He has too often been viewed as the first, or one of the first, to grasp the possibilities and promote the cause of modern mathematical physics. Even those who have noticed that Bacon was more given to the praise than to the practice of mathematics have seen in his programmatic statements an anticipation of seventeenth-century achievements. But if we judge Bacon by twentieth-century criteria and pronounce him an anticipator of modern science, we will fail totally to understand his true contributions; for Bacon was not looking to the future, but responding to the past; he was grappling with ancient traditions and attempting to apply the truth thus gained to the needs of thirteenth-century Christendom. If we wish to understand Bacon, therefore, we must take a backward, rather than a forward, look; we must view him in relation to his predecessors and contemporaries rather than his successors; we must consider not his influence, but his sources and the use to which he put them.

When Charles Darwin published his theory in 1859 the biological community gave very different receptions to the idea of evolution and to the theory of natural selection. Evolution was accepted as widely and rapidly as natural selection was rejected. Most biologists were ready to accept that evolution had occurred, but not that natural selection was its cause. They preferred other explanations of evolution, such as theories of big directed variation, or admitted that they did not know its cause. Darwin himself never maintained that natural selection was the sole cause of evolution. He thought of it as one among several causes, and did not specify how much evolution had occurred by the natural selection of fortuitous variations, and how much by other factors such as the inherited effects of use and disuse. However, Darwin did maintain that natural selection was in principle capable of explaining all the observed properties of organisms. He did not think that there were some characteristics that were particularly likely to have evolved by natural selection, and other kinds that were not. Against this, many of this critics thought that there were characteristics that natural selection was particularly powerless to explain. Thus it could not account for characteristics that were detrimental, or those that seemed useless (such as species differences), or those that were of too little importance for natural selection to have favoured them. There were also characteristics of such complexity that it was unimaginable that natural selection could have built them up in tiny stages from fortuitous, undirected variants. The present essay will be concerned with just one, the last mentioned, of these kinds of characteristics.