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.