14 TIME AND PERIODICITY 



Earth's annual revolution. When fanciful specu- 

 lators seek to imagine what kind of living beings 

 might be encountered on the other planets of our 

 system, they usually make calculations as to the 

 force of gravity on the surface of these planets 

 and conjure up from such data the possible size of 

 the inhabitants, their relative strength and agility 

 of movement, etc. So far so good. But the first 

 question we should ask, before proceeding to our 

 speculative synthesis, should rather be the length of 

 the planet's diurnal rotation and annual revolution 

 periods. Certain planets, such as Mars and Venus, 

 have rotation periods not very different from those 

 of our own Earth. 1 Other things being equal, there- 

 fore, a certain similarity of animal life must be 

 supposed possible on these planets. On the other 

 hand, the marked difference in their revolution 

 period would lead us to expect a very wide divergence 

 between their lower forms of life, if any such there 

 be, and our own terrestrial vegetation. The 

 shorter the annual period the more would the vegetal 

 approximate to the animal, and vice versa. It would, 

 however, be foolish to waste more time over a 

 speculation so remote. 



But these two facts remain unshaken : (1) That 



1 Recently, we believe, astronomers have favoured the view that 

 the day of Venus is equal in length to her year. 



