The two periodic motions which most essentially affect and concern us human beings are necessarily the two periodic motions of the globe which we inhabit--its rotation upon its axis which gives us the alternation of Day and Night, and its revolution round the Sun which gives us the year with its Seasons. To the former of these, animal life seems most directly related; to the latter, the life of the vegetal orders. It is evident that the forms of animal life on the globe are necessarily determined by the periodic law of the Earth's diurnal rotation. This accounts for the alternations of waking and sleeping, working and resting, and so forth. In like manner the more inert vitality of the vegetable kingdom is determined by the periodic law of the Earth's annual revolution. When fanciful speculators 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.[14:1] Other things being equal, therefore, 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 our measurements and whole science of Time depend absolutely on the operation throughout Nature of the Law of Periodicity, and (2) that the periodicities which affect and determine animal and vegetal life upon our Earth are the periodic movements of rotation and revolution of that Earth itself. Now it is to the curvilinear motions of the heavenly bodies that we must ascribe our subjection to the periodic law. If these heavenly bodies moved for ever in straight lines, as they would do if unacted on by natural forces, the periodic rhythm of Nature would disappear.