The dictionary defines "monotonous" as being synonymous with "wearisome." That is putting it mildly.
But notice that the learned observer defines the kind of reflection which distinguishes us from the animals as the POWER OF CONSIDERING OUR OWN MODIFICATIONS.
It is the one word that positively defines the quality of the subject being discussed, and the author of those lines desired to bring it out emphatically, as he has shown by contrasting it with another idea.