A comparison between the first and second editions indicates, but by no means expresses, his growing convictions on evolution and natural selection. Natural selection he had not discovered when the MS. of the first edition was complete; and if we had no further evidence we could not, from any passage in the work, maintain that he was convinced of evolution. His great caution in dealing with so tremendous a problem explains why the second edition does not reflect the state of his mind at the time of its publication. He tells us (“Autobiography”) that in the preparation of this second edition he “took much pains,” and we may feel confident that much of this care was given to the decision as to how much he should reveal and how much withhold of the thoughts which were occupying his mind, and the conclusions to which he had at that time arrived. That he did attribute much importance to the evolutionary passages added in the second edition is shown by his letter to Lyell (July, 1845), in which he alludes to some of them, and specially asks Lyell to read the pages on the causes of extinction. He also edited and superintended the “Zoology of the Voyage of H.M.S. _Beagle_,” the special parts of which were written by various eminent systematists, and appeared separately between 1839 and 1843. He also read several papers before the Geological Society, including two (1838 and 1840) on the Formation of Mould by the Action of Earth-Worms--a subject to which he returned, and upon which his last volume (published in 1881) was written. He also read a paper on the Parallel Roads of Glen Roy before the Royal Society (published in the _Phil. Trans._, 1839). These wonderful parallel terraces are now admitted to be due to the changes of level in a lake following those of an ice-barrier at the mouth of the valley. At the time Darwin studied them, the terraces were believed to have been formed by a lake dammed back by a barrier of rock and alluvium; this he proved to be wrong, and as no other barrier was then available--for the evidences of glaciation had not then been discovered by Agassiz--he was driven, on the method of exclusion, to the action of the sea. Upon this subject he says, in the “Autobiography,” “My error has been a good lesson to me never to trust in science to the principle of exclusion.” On January 29th, 1839, he married his cousin, Emma Wedgwood, the daughter of Josiah Wedgwood, of Maer. They resided at 12, Upper Gower Street until September 14th, 1842, when they settled at Down. The few graceful and touching words in which Francis Darwin, in the “Life and Letters,” alludes to his father’s married life show how deep is the debt of gratitude which the world owes to Mrs. Darwin; for without her constant and loving care it would have been impossible for Darwin to have accomplished his life-work. [Sidenote: ON CORAL REEFS.]