In concluding this portion of my discussion of the problem of Mars, I wish to call attention to the fact that my argument, founded upon a comparison of the physical conditions of the earth and moon with those of Mars, is dependent upon a small number of generally admitted scientific facts; while the conclusions drawn from those facts are simple and direct, requiring no mathematical knowledge to follow them, or to appreciate their weight and cogency.

I claim for them, therefore, that they are in no degree speculative, but in their data and methods exclusively scientific.

In the next chapter I will put forward a suggestion as to how the very curious markings upon the surface of Mars may possibly be interpreted, so as to be in harmony with the planet's actual physical condition and its not improbable origin and past history..