He realizes that we live in a world of imperfect knowledge.

他意识到我们生活在一个知识不完善的世界里。

Nor have I been disappointed; in this and in all other perplexing cases I have invariably found that our knowledge, imperfect though it be, of variation under domestication, afforded the best and safest clue.

我也没有失望；在这个以及所有其他令人费解的案例中， 我总是发现我们对驯化下的变异的了解虽然不完善， 但却提供了最好和最安全的线索。