64 AN INTRODUCTION TO SCIENCE 



If, when we ask the inevitable question, "Is this the only 

 planet upon which life is possible?" the astronomer or 

 spectroscopist answers, ' ' There is no other in which proto- 

 plasm would remain a compound, or in which it would find 

 itself in the presence of oxygen ;" then it is idle to speculate 

 as to whether life is possible elsewhere than on the earth. If 

 Venus does not rotate upon her axis, but always turns one 

 face to the sun and the other to the outer cold, there is no 

 life on Venus. If Mars is too cold for protoplasmic meta- 

 bolism, or if, as Dr. Johnstone Stoney calculates, the force 

 of gravity on this planet is too small to prevent water- 

 vapour from escaping into space, then there is no life on Mars. 

 Speculation as to the possible existence of different orders 

 of living things, of beings which do not contain nitrogen or 

 exhibit life by combining with oxygen, pierces beyond the 

 domain of science. There have not been wanting thinkers 

 who assert that they can imagine beings in whose constitu- 

 tion silicon plays the same part which nitrogen plays i:i 

 ours ; living things with the same constitution as china dolls. 

 Fancy may play at speculation in this way. It may surround 

 its new creation with an atmosphere of iodine, and feed 

 its inhabitants upon carbonate of lime. They may suffer 

 calcareous pains and give way to siliceous emotions. 

 But it is not Science. Speculation has lost touch with 

 experience. 



Living things require certain strictly limited conditions of 

 existence. Plants cannot fix carbon from the atmosphere 

 unless the temperature be somewhat above the freezing 

 point, and somewhat less than half-way to the boiling point 

 of water ; and animal life depends upon the preexistence 

 of plants. The question is therefore narrowed down to 

 this : For how long has the temperature of the earth been 

 fixed within these limits, other conditions, such as the force 

 of gravitation and the receipt of light from the sun, being 

 the same as at present? Sunshine and shower, day and 

 night, moderate heat and moderate cold were as necessary 



