How did these isolating… mechanisms develop? And that is still not yet entirely clear because as even was said in Darwin's own day, why should something be selected not to mate with the other population when they aren't in contact at all? There shouldn't be any selection… selective advantage to have an isolating mechanism in this species against this one if they are not in any contact whatsoever. Well, there are a number of ways to explain this. I will not go into detail, but in the case of two species developing out of a single population – so-called sympatric speciation – which for a long time we had no good instances that it occurred at all, but there seem to be some instances now in fresh water fishes, it is possible that this barrier developed by mate selection, in other words, by sexual selection. But the details are still obscure, but the important point at this stage is to have an open mind, to see there is a possibility and now this must be analyzed. All these were questions that went around among the naturalists who were interested in species and speciation, and at the very same time the geneticists had no interest in these problems at all. You go to… go to the classical writings of RA Fisher, of Haldane, of Sewall Wright, of HJ Muller, even of DH Morgan, and they discussed how an adaptation was acquired, they've discussed what happened within a given gene pool, but they nowhere discussed the problems of the origin of biological diversity. And this of course set up a considerable tension in the biological community because the geneticists were only interested in the happenings in a population while the naturalists were interested in the business of the biological diversity, the origin of new species, the origin and the development of these species into more different, higher taxa etc. And there was a third group of… of biologists interested in evolution, which dealt with still other problems. And those were the palaeontologists who were told that speciation, from Darwin on, that speciation and evolution was a gradual process, and yet when they looked at the fossil record all they found were just discontinuators. New species seemed to originate very suddenly; they suddenly turned up in the fossil record all of a sudden and so on and so forth. So people were arguing against each other, and there were still a lot of Lamarckians and there… there seemed to be no way in which these differences of opinions could be resolved and solutions could be found that would bring these various feuding kinds of evolutionists together. And this was the situation right up till about 1935. I know a paper in 1935 where somebody said, ‘After I've looked at the literature, there is no question that there will be no consensus in the foreseeable future’. And yet in 1937, a book was published by the Russian geneticist, naturalist, Theodosius Dobzhansky, at that time at Caltech [California Institute of Technology] in California, on the basis of lectures he had given… the Jesup lectures he had given at Columbia University, this book… immediately everybody agreed, well, here is the basis for such a consensus.