Darwin did eventually make his belief in our kinship with the apes explicit in The Descent of Man in 1871. The conclusion was a bold one since nothing in the fossil record supported such a notion. The only known early human remains of that time were the famous Neandertal bones from Germany and a few uncertain fragments of jawbones, and many respected authorities refused to believe even in their antiquity. The Descent of Man was altogether a more controversial book, but by the time of its appearance the world had grown less excitable and its arguments caused much less of a stir.
在1871年完成的《人的由来》一书中，达尔文最终明确阐明了人与猿的亲缘关系，这一结论是非常大胆的，因为当时还没有任何化石记录支持这样的观点。那时已经发现的早期人类化石只有在德国发现的著名的尼尔德特人骸骨，以及几块不完整的颌骨，许多有影响的权威甚至对它们是否是古人类的化石也表示怀疑。《人的由来》总体来说是一部比《物种起源》更容易引起争议的著作，但当它问世时，人们已经不那么容易激动，书里的观点所引发的讨论也远不如以前那么激烈。

For the most part, however, Darwin passed his twilight years with other projects, most of which touched only tangentially on questions of natural selection. He spent amazingly long periods picking through bird droppings, scrutinizing the contents in an attempt to understand how seeds spread between continents, and spent years more studying the behavior of worms. One of his experiments was to play the piano to them, not to amuse them but to study the effects on them of sound and vibration. He was the first to realize how vitally important worms are to soil fertility. "It may be doubted whether there are many other animals which have played so important a part in the history of the world," he wrote in his masterwork on the subject, The Formation of Vegetable Mould Through the Action of Worms (1881), which was actually more popular than On the Origin of Species had ever been.
然而，达尔文将自己晚年的大部分时间用于其他学科的研究，其中绝大部分学科只是略为触及自然选择问题。他花费了不少时间收集鸟类粪便，用以研究种子是怎样从一个大陆传到另一个大陆的，又用了几年时间研究蚯蚓的行为。他曾为蚯蚓弹钢琴，不过并不是为了愉悦它们，而是为了研究声音和震动对它们的影响。他第一个发现蚯蚓对于肥沃土壤起着至关重要的作用。“在世界历史上，很难找到很多比蚯蚓起更加重要作用的动物了。”在那本实际上比《物种起源》传播更广的著作《由蚯蚓活动形成的壤土腐殖层》（1881年）中，他曾这样写道。