As years passed, Darwin’s relation to his children reached its climax of comradeship in the constant assistance they gave him in his work. His daughter helped him clerically, and his sons, who had scientific interests of their own, participated actively and most profitably in his labors. Sometimes he made use of their keen wits to sharpen and clarify his: ‘Two of my grown-up children who are acute reasoners have two or three times at intervals tried to prove me wrong; and when your letter came they had another try, but ended by coming back to my side.’[220] Whatever the nature of the assistance might be, Darwin was always profoundly grateful for it, and his children speak particularly of the simple, humble fashion in which his gratitude was expressed. It was a pleasure to help him in any way, because you were sure that the help would be used as you meant it and would be thoroughly appreciated. And in general I do not know that the beauty of Darwin’s relation to his children can be better expressed than in the words of his son, equally honorable to son and to father: ‘I do not think his exaggerated sense of our good qualities, intellectual or moral, made us conceited, as might perhaps have been expected, but rather more humble and grateful to him. The reason being no doubt that the influence of his character, of his sincerity and greatness of nature, had a much deeper and more lasting effect than any small exaltation which his praises or admiration may have caused to our vanity.’[221] IV Though Darwin’s social activity was necessarily restricted by his ill-health, his devotion to special friends was as sweet and notable as his devotion to his family. Indeed, friendship, the natural turning to sympathetic spirits, and clinging to them with constant loyalty, seems to have been a peculiarly profound and powerful instinct in him. He took a deep interest in all his friends’ affairs, and poured out all his own interests to them with intimate and appealing effusiveness. In writing of his grandfather, he says: ‘There is, perhaps, no safer test of a man’s real character than that of his long continued friendship with good and able men.’[222] Assuredly, if the test is applied to himself, he bears it nobly. Of his longing for friendship and great aptness for it in boyhood he speaks very positively: ‘I had many friends amongst the schoolboys, whom I loved dearly, and I think that my disposition was then very affectionate.’[223] His son says that the friendships of mature life had not quite the zest and passion of those of youth; but the son adds with justice that no one who reads his father’s letters can feel that his later affections were lacking in intensity or depth.[224]