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THE POPULAR SCIENCE MONTHLY.— SUPPLEMENT. 



same degree of heat suited their requirements. 

 If; then, we find reason for believing that now 

 the inherent heat of the planet is much less than 

 that of our own earth, so that on this account 

 the descendants of those creatures would be un- 

 able to exist unless great modifications had taken 

 place in their requirements, which modifications 

 seem outside any effects which could be attrib- 

 uted to natural selection, then the inference that 

 therefore these races of creatures have died out 

 is certainly strengthened, and in no small degree, 

 by the fact that the supply of heat received from 

 the sun is much smaller in the case of Mars than 

 in the case of our earth. Seeing, then, that the 

 average daily supply of light and heat on Mars 

 (taking square mile for square mile of his sur- 

 face) is less than the average daily supply on our 

 earth in the proportion of two to five, we have 

 here a strong argument, we will not say in favor 

 of the belief that Mars is not now inhabited, but 

 in favor of the belief that the duration of the 

 life-supporting era has been, is, or will be, much 

 more rapidly shortened than in our earth's case, 

 by the cooling of his globe. For the life-destroy- 

 ing influence of the cooling is much more effec- 

 tively strengthened in his case than in our earth's, 

 by the effect of distance from the central source 

 of light and heat. 



All the other circumstances in the condition 

 of Mars point directly to the conclusion that Mars 

 must have long since passed his planetary prime. 

 His orbit being outside the earth's, he was prob- 

 ably formed far earlier, though this is not so cer- 

 tain as it was held to be when Laplace's nebular 

 theory was first advanced. It is, however, very 

 unlikely that he began to be formed later; and, 

 as he is much smaller, he would probably be 

 fashioned more quickly. It is still more probable, 

 in fact very much more probable, that he cooled 

 much more quickly than the earth. His mass is 

 not much more than a ninth of hers, while his 

 surface is only about one-third of hers. He had, 

 then, originally, even if of the same temperature 

 when first formed, only one-ninth her amount of 

 heat to distribute, so that if he had radiated away 

 at one-ninth of her rate, the supply would have 

 lasted as long. Pouring it away at one-third of 

 her rate — for the radiation taking place from the 

 surface is proportional to the surface — he parted 

 with it three times faster than he should have 

 done in order to cool at the same rate as the 

 earth. Hence he cooled three times faster than 

 the earth, and must have attained a condition 

 which she will not attain until three times as long 

 an interval has elapsed from the era of her first 



existence, than has already elapsed. Since most 

 geologists assign many hundred millions of years 

 to the last-named period, and all agree that it 

 must be measured by many millions of years, it 

 follows that twice as many hundred millions of 

 years must elapse if the former are right, but 

 only twice as many millions of years if the latter 

 are right, before our earth will be in the same 

 condition as Mars. In reality our aigument is 

 not at all affected by the difference of opinion 

 among geologists in this respect. For the ques- 

 tion is of the condition of Mars, not of the num- 

 ber of years which may have elapsed since he was 

 in the same condition as our earth, or of the num- 

 ber of years which may have to pass before our 

 earth will be in the same condition as Mars. 

 Whether Mars requires hundreds of millions, or 

 millions, or only thousands of years, to pass 

 through one stage of its planetary existence, our 

 earth requires about three times as long; and, 

 taking the entire development of Mars and the 

 earth (assuming they began planetary existence 

 together), Mars must be some three times as far 

 on the way toward planetary decrepitude and 

 death as our earth. 



Only one circumstance in the discussions of 

 geologists on the question of the time required 

 for the development of a globe like our earth, 

 bears very strongly on our opinion as to the ex- 

 istence of life on Mars. It is not altogether 

 certain that the life-bearing era of a planet 

 is exceedingly short compared with the era 

 of growth and preparation, and the era of de- 

 crepitude and death. So far, indeed, as astro 

 nomical considerations are concerned, we per- 

 ceive that the fashioning of a planet must be 

 a process requiring an enormous length of 

 time. The slow aggregation of nebulous mat- 

 ter, the separation of ring from ring, the break- 

 ing up of a ring into separate nebulous masses, 

 and the gathering of each ring of them into a 

 single mass, must have proceeded very slowly ; 

 and few who consider all the circumstances of 

 the case will doubt that hundreds of millions of 

 years must have elapsed between the time when 

 first the matter of a future planet began to have 

 separate existence, and when at length it was all 

 gathered together in a single mass. But what 

 followed — the gradual contraction and cooling 

 of that mass till it became a true planet, the 

 gradual cooling of the planet until its surface be- 

 came separable into land and water, the further 

 cooling till life became possible, the progression 

 of life through all its various stages till earth 

 and sea and air had each their various races of 



