Sentence:  But the "Houyhnhnms" train up their Youth to Strength, Speed, and <WSD>Hardiness</WSD>, by exercising them in running Races up and down steep Hills, and over hard and stony Grounds ... 
Meaning of the word marked with <WSD> tags:  The quality of being able to withstand fatigue and hardship; ("of a plant") the quality of being resistant to cold or other environmental conditions.