Darwin had five quite important theories which represented his evolutionary thought; these theories were not all accepted by the Darwinians, by his followers. In fact, some of them accepted only one or two of them, and it wasn't always the same which of these five theories they would accept, by the various different people that considered themselves Darwinians. For instance, TH Huxley and [Charles] Lyell never believed in natural selection. I think Lyell never really believed in common descent, he still had a sort of a vertical feeling about all this… all descent and phylogeny. Well let me go and discuss these various theories one by one. The first one was simply the fact of evolution which at the time of Darwin was still very much of a minority opinion in spite of [Jean-Baptiste] Lamarck and [Robert] Chambers and a few other forerunners. Darwin called it the theory of the non-constancy of species, because he fully realized that the species was the crucial factor in evolution. Now that's the first theory. The second one was that of common descent. Up to that time, following the scala naturae of the 18^th century, the feeling was that all life started with some very simple… animals, little infusorians or whatever they called them. Lamarck, for instance, believed that, which originated by spontaneous generation and then gradually became more and more perfect, as they called it, until finally they became vertebrates and… and right up to man. Now, this singular… single lineage ascent was widespread until Darwin's time, and Darwin was the first one who really consistently and deliberately believed in branching evolution. Lamarck recognized a few branches, but basically his concept of… evolution was just a vertical lineage. So, branching common descent, which means branching evolution, was the second of Darwin's major theories, and it was the most successful of all of them in Darwin's time. First of all, it provided by far the best evidence for evolutionary change, and secondly it provided an explanation for much of the empirical evidence that people had at that time. For instance, the evidence for the Linnaean hierarchy, why is there such a hierarchy? And the findings of [Georges] Cuvier and the comparative anatomists that most of the organisms could be… or certainly most of the animals could be assigned to a number of types and good deal of development within these types. All this was at once fully explained by the theory of common descent. The third of Darwin's theory was that evolution was gradual. It wasn't a… by jumps, by mutations, by saltations from one type to another as the essentialists had postulated because an… an essence cannot change gradually, and Darwin introducing population thinking saw that all evolution was gradual. And of course this was reinforced or maybe even initiated by Lyell's uniformitarian theories of geology which also emphasized that all change was gradual. Now that was the third. The fourth one was theory of speciation; that species multiply and that species can give rise to new species. And the fifth one, of course, was his theory of natural selection, which in Darwin's lifetime and up to 1930s was in a minority view when more people believed that evolution was due to Lamarckian forces or due to an intrinsic drive toward perfection, in other words teleological thinking, or due to… jumps, mutations and so forth. So the… these opposing theories were more popular than Darwin's theory of natural selection.