370 CHANGES BY TIME. 



as varied as these are ; and when a change takes 

 place in either of these, if the living productions 

 cannot alter their habits so as to accommodate 

 themselves to the change, there is no alternative, 

 but they must perish. Also, though we know 

 nothing about the primary germs of plants or of 

 animals, till we find them developed in visible and 

 tangible embryos, we must not make our igno- 

 rance the measure of nature's working; for though 

 we have seen them come only in one way, and, 

 generally speaking, perish in another, we are not 

 to suppose that that is all. When forests of one 

 kind of timber are cut down, new plants make 

 their appearance ; and there are evidences that 

 races, both of plants and of animals, have perished 

 even in our own country, without any great con- 

 vulsion of nature, for their remains have been left 

 on the strand, and buried by the slow progress of 

 the depositation of the matter washed down by 

 the rains, and staid by the reaction of the waves. 



Cultivation itself will deteriorate, and in time 

 destroy races, if the same race, and the same mode 

 of culture be pursued amid general change. Our 

 own times are times of very rapid change, and, 

 upon the whole, of improvement; we dare not, 

 without the certainty of their falling off, continue 

 the same stock and the same seed corn, season 

 after season, and age after age, as was done by 

 our forefathers. The general change of the coun- 

 try, must have change and not mere succession, 

 in that which we cultivate ; and thus we must 

 cross the breeds of our animals, and remove the 

 seeds and plants of our vegetables from district to 

 district. 



There is something of the same kind in human 



