34 TERRESTRIAL ADAPTATIONS. 



appear to follow any rule by which they are con- 

 nected with the distance from the snn. Mercury, 

 Venus, and Mars have days nearly the length of 

 ours. Jupiter and Saturn revolve in about ten 

 hours each. For any thing we can discover, the 

 earth might have revolved in this or any other 

 smaller period ; or we might have had, without 

 mechanical inconvenience, much longer days 

 than we have. 



But the terrestrial day, and consequently the 

 length of the cycle of light and darkness, being 

 what it is, we find various parts of the constitu- 

 tion both of animals and vegetables, which have 

 a periodical character in their functions, corres- 

 ponding to the diurnal succession of external 

 conditions ; and we find that the length of the 

 period, as it exists in their constitution, coincides 

 with the length of the natural day. 



The alternation of processes which takes place 

 in plants by day and by night is less obvious, 

 and less obviously essential to their well-being, 

 than the annual series of changes. But there are 

 abundance of facts which serve to show that such 

 an alternation is part of the vegetable economy. 



In the same manner in which Linnaeus pro- 

 posed a Calendar of Flora, he also proposed a 

 Dial of Flora, or Flower- Clock ; and this was to 

 consist, as will readily be supposed, of plants, 

 which mark certain hours of the day, by opening 

 and shutting their flowers. Thus the day-lily 



