An attempt to explain the foregoing laws of geographical distribution, on the theory of allied species having a common descent Improbability of finding fossil forms intermediate between existing species CHAPTER VII 198-213 ON THE NATURE OF THE AFFINITIES AND CLASSIFICATION OF ORGANIC BEINGS. Gradual appearance and disappearance of groups What is the Natural System? On the kind of relation between distinct groups Classification of Races or Varieties Classification of Races and Species similar Origin of genera and families CHAPTER VIII 214-230 UNITY OF TYPE IN THE GREAT CLASSES; AND MORPHOLOGICAL STRUCTURES. Unity of Type Morphology Embryology Attempt to explain the facts of embryology On the graduated complexity in each great class Modification by selection of the forms of immature animals Importance of embryology in classification Order in time in which the great classes have first appeared CHAPTER IX 231-238 ABORTIVE OR RUDIMENTARY ORGANS. The abortive organs of Naturalists The abortive organs of Physiologists Abortion from gradual disuse CHAPTER X 239-255 RECAPITULATION AND CONCLUSION. Recapitulation Why do we wish to reject the Theory of Common Descent? Conclusion INDEX 257 Portrait _frontispiece_ Facsimile _to face_ p. 50 INTRODUCTION We know from the contents of Charles Darwin's Note Book of 1837 that he was at that time a convinced Evolutionist{1}. Nor can there be any doubt that, when he started on board the _Beagle_, such opinions as he had were on the side of immutability. When therefore did the current of his thoughts begin to set in the direction of Evolution? {1} See the extracts in _Life and Letters of Charles Darwin_, ii. p. 5. We have first to consider the factors that made for such a change. On his departure in 1831, Henslow gave him vol. I. of Lyell's _Principles_, then just published, with the warning that he was not to believe what he read{2}. But believe he did, and it is certain (as Huxley has forcibly pointed out{3}) that the doctrine of uniformitarianism when applied to Biology leads of necessity to Evolution. If the extermination of a species is no more catastrophic than the natural death of an individual, why should the birth of a species be any more miraculous than the birth of an individual? It is quite clear that this thought was vividly present to Darwin when he was writing out his early thoughts in the 1837 Note Book{4}:-- "Propagation explains why modern animals same type as extinct, which is law almost proved. They die, without they change, like golden pippins; it is a _generation of species_ like generation _of individuals_."