122 Dynamic Theory. 



due to molecular motion, its periodicity to molar or motion in the mass 

 planetary motions. The most obvious of these is the alternation of 

 night and day by the periodic revolution of the earth on its axis. The 

 effect of this on plants is to periodize the action of light upon Chloro- 

 phyll. All green leafed plants, or plants containing chlorophyll, in the 

 day time decompose carbonic acid and form starch to be carried into 

 their tissues. But in the night the sunlight, which really does this work 

 for the plant, being absent, the process of making starch is suspended. 

 Daylight is used by the plant to procure the means of livelihood and 

 during the night the material secured is turned into tissue. The same 

 is the case with the animal, although not in the same proportion. On 

 account of his constant movements "and work done in the day time he 

 constantly consumes his tissues and gives them off to the air in the 

 shape of carbonic dioxide. But the greater part of this waste is due to 

 the external work he does, while the waste which takes place in the 

 night is due to the internal work of repair, just as it is in plants. 

 All work is done at the expense of tissue. External work is generally 

 done during consciousness, but whether it all is or not, at any rate con- 

 sciousness itself is work (as will be shown elsewhere). It follows 

 therefore that repairs cannot go on so well during consciousness as dur- 

 ing unconsciousness. Therefore under the law of selection, there would 

 be a tendency to reduce animals to unconsciousness during repairs. 

 Hence we have sleep, or the state of unconsciousness which is necessary 

 to the discontinuance of work, whereby the general energy of the organ- 

 ism is devoted to the repairs of wasted tissue. The causes that fix 

 night as the general time for sleep are as obvious in the case of animal 

 life as in that of vegetable, although not precisely the same, except per- 

 haps to a limited extent. The exceptions are all accounted for in ac- 

 cordance with the law, which is that light is productive of work, proven 

 in the case of plants being awake at night in presence of artificial light 

 and sleeping in the day time when confined in artificial darkness. 



The revolution of the earth around the sun, which coupled with the 

 inclination of the polar axis produces the alternations of the seasons, is 

 another periodical movement no less momentous than the first. By this, 

 in the temperate zones a period is reached every year in which the tem- 

 perature is too low to allow of the chemical reactions required for 

 growth, viz. , the decomposition by light of carbon dioxide and the de- 

 composition of starch and sugar by diastase in the tissues of plants. 

 During the warm months, therefore, myriads of herbs pass entirely 

 through the terms of their cycle of existence, beginning with seed, con- 

 tinuing through germination, growth, fertilization and maturation of 

 the new seed. These are called annual plants. Others are biennial, 

 growing one season, stopping during winter and forming their seeds the 



