Final answer:
The canals observed on Mars were optical illusions caused by the human mind's tendency to see order in random features.
Step-by-step explanation:
The canals that late 19th and early 20th-century observers thought they saw on Mars turned out to be optical illusions. These straight lines were the result of the human mind's tendency to see order in random features that are glimpsed dimly at the limits of the eye's resolution. When small, dim dots of surface markings are connected by our minds, they appear as straight lines.