148 EELATIVE GEOLOGICAL ANTIQUITY OF TEEES. 



features in plants which have not been affected by the physi- 

 cal revolutions which the earth has undergone. Surely there 

 is nothing- unreasonable in believing, that in such cases there 

 is, to a certain extent, power given to vegetable matter to 

 adapt itself to a gradual change of circumstances. We know 

 that great changes may be effected in a brief space of time in 

 the organization of plants by cultivation, or a change in their 

 external circumstances, and why should not an organic 

 change be brought about in plants when their external 

 circumstances are altered by Nature in the course of ages ? 



" If," says Prof. Draper,* " we expose some spring water 

 to the sunshine, though it may have been clear -and transpa- 

 rent at first, it presently begins to assume a greenish tinge, 

 and after a while flocks of green matter collect on the sides 

 of the vessel in which it is contained ; in these flocks, when- 

 ever the sun is shining, bubbles of gas may be seen, which 

 if collected prove to be a mixture of oxygen and nitrogen, 

 the proportions of the two being variable. In the mean time 

 the green matter rapidly grows ; its new parts, as they are 

 developed, being all day long covered with air bells, which 

 disappear as soon as the sun has set. Similar green flocks 

 to these of which we speak, are also found on the surfaces 

 of rocks exposed to the sea, damp walls, and other places 

 where there is constant moisture. These plants belong to the 

 Alga3 or sea- weed tribe. This green matter thus formed is 

 produced from the gaseous matter, Oxygen, Nitrogen, and 

 Carbonic acid, contained in the water in a state of solution. 

 It is necessary, however, to admit the existence of some germ 

 or objective-point on which the light can act. As we have 

 already said, a bubble of gas soon makes its appearance, and 

 growth with the development of the green color takes place. 

 If we examine the changes now occurring in the water, we 

 find that the Carbonic acid is disappearing and Oxygen and 

 Nitrogen are evolved. Contemporaneously with the develop- 

 ment of this green matter that of animal life begins, and thou- 

 sands of aquatic animals are formed which live on it as food. 



* " A Treatise on the Forces which produce the Organization of Plants," 

 by JOHX W. DRAPER. 1844. 



