Martian climate; effects which must be influenced by conditions of mountain elevation, atmospheric currents, etc.  We, thus, follow Lowell in ascribing the accentuation of the canals to the circulation of water in Mars; but we assume a simple and natural mode of conveyance and do not postulate artificial structures of all but impossible magnitude.  That vegetation may take part in the darkening of the elevated tracts is not improbable.  Indeed we would expect that in the Martian climate these tracts would be the only fertile parts of the surface.

Clouds also there certainly are.  More recent observations

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appear to have set this beyond doubt.  Their presence obviously brings in other possible explanations of the coming and going of elevated surface features.

Finally, we may ask what about the reliability of the maps?  About this it is to be said that the most recent map—­that by Lowell—­has been confirmed by numerous drawings by different observers, and that it is,itself the result of over 900 drawings.  It has become a standard chart of Mars, and while it would be rash to contend for absence of errors it appears certain that the trend of the principal canals may be relied on, as, also, the general features of the planet’s surface.

The question of the possibility of illusion has frequently been raised.  What I have said above to a great extent answers such objections.  The close agreement between the drawings of different observers ought really to set the matter at rest.  Recently, however, photography has left no further room for scepticism.  First photographed in 1905, the planet has since been photographed many thousands of times from various observatories.  A majority of the canals have been so mapped.  The doubling of the canals is stated to have been also so recorded.[1]

The hypothesis which I have ventured to put before you involves no organic intervention to account for the

[1] E. C. Slipher’s paper in Popular Astronomy for March, 1914, gives a good account of the recent work.

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details on Mars’ surface.  They are physical surface features.  Mars presents his history written upon his face in the scars of former encounters—­like the shield of Sir Launcelot.  Some of the most interesting inferences of mathematical and physical astronomy find a confirmation in his history.  The slowing down in the rate of axial rotation of the primary; the final inevitable destruction of the satellite; the existence in the past of a far larger number of asteroids than we at present are acquainted with; all these great facts are involved in the theory now advanced.  If justifiably, then is Mars’ face a veritable Principia.

To fully answer the question which heads these lectures, we should go out into the populous solitudes (if the term be permitted) which lie beyond our system.  It is well that there is now no time left to do so; for, in fact, there we can only dream dreams wherein the limits of the possible and the impossible become lost.