In Gulliver's Travels, what does Gulliver conclude about the difference between Yahoos and man?
In his final journey, Gulliver comes upon the island of the Houyhnhnms, intelligent horses, and Yahoos, de-evolved human creatures with no intellect. While staying with the Houyhnhnms, Gulliver comes to the realization that Humankind, from which he descends, are barely more intelligent than the barbaric Yahoos, but have enough intellect to do great harm.
When I thought of my family, my friends, my countrymen, or the human race in general, I considered them, as they really were, Yahoos in shape and disposition, perhaps a little more civilized, and qualified with the gift of speech; but making no other use of reason, than to improve and multiply those vices whereof their brethren in this country had only the share that nature allotted them.
(Swift, Gulliver's Travels, eNotes eText)
In other words, while Humankind shares the baser qualities of the Yahoos, they only share the barest semblance of reason with the Houyhnhnms, and so are inferior. Their slight civilization and speech allow them to pretend to heights of reason and intellect, when in fact they use these gifts for little other than war and strife. Gulliver, disgusted with himself and his own previous association with the Yahoos of Humanity, withdraws from society and finds himself unable to live with even his beloved wife, although he tries to slowly acclimate himself to her. In this manner, Gulliver plans to spend the rest of his life teaching his children, and by proxy the rest of Humankind, the wiser teachings of the Houyhnhnms in an attempt to raise the levels of Human consciousness and reason.