22 TERRESTRIAL ADAPTATIONS. 



operate upon plants is completed. There is also 

 in plants a cycle of internal functions, corre- 

 sponding to this succession of external causes. 

 The length of either of these periods might have 

 been different from what it is, according to any 

 grounds of necessity which we can perceive. 

 But a certain length is selected in both instances, 

 and in both instances the same. The length of 

 the year is so determined as to be adapted to the 

 constitution of most vegetables ; or the construc- 

 tion of vegetables is so adjusted as to be suited to 

 the length which the year really has, and unsuited 

 to a duration longer or shorter by any consider- 

 able portion. The vegetable clock-work is so set 

 as to go for a year. 



The length of the year or interval of recurrence 

 of the seasons is determined by the time which 

 the earth employs in performing its revolution 

 round the sun : and we can very easily conceive 

 the solar system so adjusted that the year should 

 be longer or shorter than it actually is. We can 

 imagine the earth to revolve round the sun at a 

 distance greater or less than that which it at 

 present has, all the forces of the system remaining 

 unaltered. If the earth were removed towards 

 the centre by about one-eighth of its distance, the 

 year would be diminished by about a month ; and 

 in the same manner it would be increased by a 

 month on increasing the distance by one-eighth. 

 We can suppose the earth at a distance of 84 or 



