Although, as already remarked, it seems necessary to assume that any life existing in the moon prior to its great volcanic outburst must have ceased at that time, yet the possibility may be admitted that life could reappear upon the moon after its surface had again become quiet and comparatively undisturbed.  Germs of the earlier life might have survived, despite the terrible nature of the catastrophe.  But the conditions on the moon at present are such that even the most confident advocates of the view that the lunar world is not entirely dead do not venture to assume that anything beyond the lowest and simplest organic forms—­mainly, if not wholly, in the shape of vegetation—­can exist there.  The impression that even such life is possible rests upon the accumulating evidence of the existence of a lunar atmosphere, and of visible changes, some apparently of a volcanic character and some not, on the moon’s surface.

Prof.  William H. Pickering, who is, perhaps, more familiar with the telescopic and photographic aspects of the moon than any other American astronomer, has recorded numberless instances of change in minute details of the lunar landscapes.  He regards some of his observations made at Arequipa as “pointing very strongly to the existence of vegetation upon the surface of the moon in large quantities at the present time.”  The mountain-ringed valley of Plato is one of the places in the lunar world where the visible changes have been most frequently observed, and more than one student of the moon has reached the conclusion that something very like the appearances that vegetation would produce is to be seen in that valley.

Professor Pickering has thoroughly discussed the observations relating to a celebrated crater named Linne in the Mare Serenitatis, and after reading his description of its changes of appearance one can hardly reject his conclusion that Linne is an active volcanic vent, but variable in its manifestations.  This is only one of a number of similar instances among the smaller craters of the moon.  The giant ones are evidently entirely extinct, but some of the minor vents give occasional signs of activity.  Nor should it be assumed that these relatively slight manifestations of volcanic action are really insignificant.  As Professor Pickering shows, they may be regarded as comparable with the greatest volcanic phenomena now witnessed on the earth, and, speaking again of Plato, he says of its evidences of volcanic action: 

“It is, I believe, more active than any area of similar size upon the earth.  There seems to be no evidences of lava, but the white streaks indicate apparently something analogous to snow or clouds.  There must be a certain escape of gases, presumably steam and carbonic acid, the former of which, probably, aids in the production of the white markings."[19]

[Footnote 19:  Annals of Harvard College Observatory, vol. xxxii, part ii, 1900.]