PARATHEHMIC RAYS. 377 



relatively to cacli other, subject to an interesting change. 

 In the spring, the actinic or chemical power prevails, 

 and during this period its agency is required for the 

 vitalization of the germ. As the summer comes on, 

 the actinic rays dimmish, and those of light increase. 

 Perhaps it would be more strictly correct to say that 

 the luminous intensity being increased, the chemical 

 power was retarded ; the former expression implies a 

 variation in quantity, which may not be correct. "We 

 see the necessity for this, since luminous power is re- 

 quired for the secretion of the carbon, with which the 

 woody fibre is formed, and also for the elaboration of 

 the proximate principles of the plant. Autumn, the 

 season of fruit, is characterized by an increase of the 

 heat rays, and a diminution of the others : this change 

 being, necessary, as science now teaches us, for the due 

 production of flower and fruit. 



The calorific rays of the solar beam, to which the 

 autumnal phenomena of vegetation appear particularly 

 to belong, are of a peculiar character. They have been 

 called the Parathermic rays, and exhibit a curious 

 compound nature. To these rays we may refer the 

 ripening of fruit and grain, and the browning of the leaf 

 before its fall. May not the rise of the sap in spring 

 be traced to the excitement of either light or actinism, 

 and its recession, in the autumn, to that power from 

 which the plant is found to bend, and which appears to 

 be their modified form of heat ? 



There can be no doubt that the varieties of climate 

 and the peculiarities of countries, as it regards their 

 animal and vegetable productions, are dependent on the 

 same causes. The distribution of species has been 

 referred by some to specific centres of creation around 

 which the plants and animals have spread, without 

 reference to physical conditions. Although centres of 

 creation may be admittedi, these centres themselves 

 have been determined by the physical fitness of each 



