What was Lowell's primary theory regarding the function of the Martian canals?

Answer

They transported water from melting polar ice caps to warmer equatorial regions.

Lowell theorized that Martians engineered these conduits as a necessity for survival, moving water from the poles to the warmer equatorial regions where their civilization was thought to reside, due to the planet drying up.

What was Lowell's primary theory regarding the function of the Martian canals?
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