But in the 'Origin of Species' itself Darwin reiterated and emphasised Knight's law as a general and all-pervading principle of nature, placing it at the same time on broader and surer biological foundations by affiliating it intimately upon his own great illuminating and unifying doctrine of natural selection. He also soon after rescued from oblivion Sprengel's curious and fairy-like book, showing in full detail in his work on orchids the wonderful contrivances by which flowers seek to attract and to secure the assistance of insects for the impregnation of their embryo seeds. In the 'Variation of Animals and Plants under Domestication,' he further showed that breeding in-and-in diminishes the strength and productiveness of the offspring; while crossing with another stock produces, on the contrary, the best possible physical results in both directions. And now at last, in the 'Effects of Cross and Self fertilisation,' he proved by careful and frequently repeated experiments that a constant infusion of fresh blood (so to speak) is essential to the production of the healthiest offspring. In the words of his own emphatic summing up, 'Nature abhors perpetual self-fertilisation.' The immediate result of these new statements and this fresh rationale of Knight's law was to bring down Sprengel forthwith from the top shelf, where he had languished ingloriously for seventy years, and to set a whole school of ardent botanical observers working hard in the lines he had laid down upon the mutual correlations of insects and flowers. A vast literature sprang up at once upon this enchanting and long-neglected subject, the most eminent workers in the rediscovered field being Delpino in Italy, Hildebrand and Hermann Müller in Germany, Axel in Sweden, Lubbock in England, and Fritz Müller in tropical South America. Darwin found the question, in fact, almost taken out of his hands before he had time himself to treat of it; for Hildebrand's chief work was published as early as 1867, while Axel's appeared in 1869, both of them several years earlier than Darwin's own final essay on the subject in the 'Effects of Cross and Self Fertilisation.' No statement, perhaps, could more clearly mark the enormous impetus given to researches in this direction than the fact that D'Arcy Thompson, in his appendix to Müller's splendid work on the 'Fertilisation of Flowers,' has collected a list of no less than eight hundred and fourteen separate works or important papers bearing on that special department of botany, almost all of them subsequent in date to the first publication of the 'Origin of Species.' So widely did the Darwinian wave extend, and so profoundly did it affect every minute point of biological and psychological investigation.