i62 NATURAL SCIENCE. Sept., 



are similar in many respects. Both the earth and Mars " turn 

 alternate cheeks to the reverberation of the same blazing world," and 

 both encounter the same, or nearly the same, range of temperature. 

 Mars, like the earth, is covered by land and by sea. It has moun- 

 tain ranges and volcanoes, plains and valleys ; it has wind storms 

 and rain and clouds. The polar regions are covered with snow and 

 ice. Summer and winter succeed each other as here, although the 

 actual transition from season to season is much more sudden. 

 Mr. Norman Lockyer, for instance, says that he has seen a tract 

 of snow, that would reach from London to Madrid, melt in a week, 

 and suggests that the so-called canals of Mars are deep channels 

 hollowed out by the sudden periodical melting of the snows. We 

 know, of course, that the same chemical elements, and many of the 

 compounds with which we are familiar here, exist on Mars. The 

 only known physical condition differing much is the force of 

 gravity, which is considerably less on Mars. 



Can we conclude anything from these data about the presence 

 of organic life ? In the first place, organic life, as we know it, 

 is associated invariably with protoplasm. In that mixture of 

 materials, the chief chemical compounds present are the albuminous 

 substances known as proteids. A temperature about freezing point 

 arrests the activity of these : a temperature above 40 degrees centi- 

 grade destroys them. So far there is no difficulty, for certainly 

 there are on Mars, as here, some parts where the temperature remains 

 all the year round within these limits. Next, water must be present, 

 and in this respect also, the conditions on Mars almost certainly 

 are favourable. The water must be tolerably pure : the presence 

 of a certain amount of salts would be no barrier, for, although even 

 the best elementary text-books of biology know only the amcebae of 

 fresh water, amoebae are common animals of brackish water, and 

 of the sea-shore. The presence of free mineral acids in solution 

 would be fatal : but from the similar physical and chemical conditions 

 here and on Mars, there is every reason to believe that free acids 

 would exist as seldom in nature there as here. The different con- 

 ditions of gravity afford more perplexing problems. Oscar Hertwig 

 and others, by experiments upon developing ova in centrifugal 

 machines, and so forth, have shown that neutralising the effects of 

 gravity makes little difference to the forces of growth. But the 

 researches of Biitschli open up graver questions. If there be truth 

 in his analogy between protoplasm and physical foams, only those 

 who have attempted to repeat his experiments can judge how small 

 are the differences in tensions and consistencies that make or mar 

 the results. If the structures and movements of protoplasm depend, 

 as he thinks, chiefly upon differences of surface tension, no one can 

 say how a difference in the force of gravity would interfere with the 

 delicate interaction of forces upon which tensions depend. 



But granted the possibility of the mere existence of protoplasm 



