CONDITIONS OF HABITABILITY OF A PLANET. 79 



for the presence of beings that we could recognize as being 

 essentially of the same order as ourselves, possessing an intelli- 

 gent spirit lodged in an organic body. Animals without 

 intelligence could not be dignified by the title of " inhabitant," 

 nor could disembodied intelligences, such as men have fabled to 

 live in rocks, or streams, or trees — fairies, nymphs and elves 

 and the like — be accurately described by the same term. We 

 may readily imagine that in outward form the inhabitants of 

 another world might differ very greatly from ourselves, but, like 

 us, they must be possessed of intelligence and self-consciousness, 

 and these qualities must be lodged in and expressed by a living, 

 material body. Our inquiry is a physical one ; it is the necessi- 

 ties of the living body that must guide us in it ; a world 

 unsuited for living organisms is not, in our sense, a habitable 

 world. 



What constitutes a living organism ? It is almost impossible 

 to give a comprehensive and satisfactory definition, yet we all 

 know some of the chief characteristics of an organism. In the 

 first place it is a machine. Like man-made machines it is a 

 storehouse of energy, but it differs from artificial machines in 

 that, of itself and by itself, it is continually drawing non-living 

 matter into itself, converting it into an integral part of the 

 organism, and so endowing it with the qualities of life, and it 

 derives from this non-living matter fresh energy for the 

 carrying on of the work of the machine. The living organism, 

 therefore, is continually changing its substance, while it remains 

 as a whole essentially the same. As Professor S. J. Allen has 

 remarked : " The most prominent and perhaps the fundamental 

 phenomenon of life is what may be described as the energy 

 traffic, or the function of tracling in energy. The cliief physical 

 function of living matter seems to consist in absorbing energy, 

 storing it in a higher potential state, and afterwards partially 

 expending it in the kinetic or active form." 



Here is the wonder and mystery of life, the power of the 

 living organism to assimilate dead matter, to give it life, and 

 bring it into the law and unity of the organism itself. But it 

 cannot do this indiscriminately ; it is not able thus to convert 

 every dead material ; it is restricted, narrowly restricted, in its 

 action. 



First of all, living organisms are not built up out of every 

 element ; four elements must always be present and be 

 predominant ; the four being hydrogen, oxygen, nitrogen, and 

 carbon. The compounds which these four elements form with 

 each other in living organisms are most complex and varied^ 



