Also, to confirm their accuracy experiments have to be repeated. Goethe remarks with justice that it is not necessary to travel all over the world to make sure that the sky is everywhere blue,[470] and no doubt there are repetitions that are vain and superfluous. But many difficult and delicate researches have to be gone over again and again that the minutest detail may be complete; and the process may involve months of tedious delay. Darwin’s unfailing care, both to repeat and to avoid repetition, is well indicated in his son’s comment: ‘Although he would patiently go on repeating experiments where there was any good to be gained, he could not endure having to repeat an experiment which ought, if complete care had been taken, to have succeeded the first time.’[471] Here too, one cannot appreciate how immense and thorough his experimentation was without looking through such books as the ‘Animals and Plants under Domestication’ or ‘The Power of Movement in Plants.’ The endless repetition of slightly varied combinations to test a difficult or remote conclusion makes one feel how persistent and inexhaustible the patience was. And after all the years of repeated and varied research and investigation he writes the almost pathetic epilogue six months before his death: ‘I wish that I had enough strength and spirit to commence a fresh set of experiments, and publish the results, with a full recantation of my errors when convinced of them.’[429] Finally, experiments have to be not only made, but recorded, and the accuracy, so essential with all scientific observation, is above all essential here, since the omission of a link in the record may be ruinous to the continuity of the logical chain. It is peculiarly characteristic of Darwin that he wanted the record of error and mistake as well as of success. ‘I remember,’ says his son, ‘how strongly he urged the necessity of keeping the notes of experiments which failed, and to this rule he always adhered.’[430] The value of preserving and comparing apparently insignificant and meaningless notes is sometimes brought out, as in a special case of orchids: ‘I had given up the case as hopeless, until summing up my observations, the explanation presently to be given, and subsequently proved by repeated experiments to be correct, suddenly occurred to me.’[431] Long experience of his own mistakes and failures induces extreme scepticism as to the results of others: ‘_The difficulty is to know what to trust_.’[352] Even when he has taken the greatest pains, the scepticism often lingers and forces him to repeat although with an identical result: ‘Notwithstanding the care taken and the number of trials made, when in the following year I looked merely at the results, without reading over my observations, I again thought that there must have been some error, and thirty-five fresh trials were made with the weakest solution; but the results were as plainly