No. 496] ASPECTS OF THE SPECIES QUESTION 229 



This idea proceeds on the supposition that the species 

 which we know have existed as long- as the earth has had 

 its present form. Xo doubt there were, in the preceding 

 state of our globe, other species of plants, which have 

 now perished, and the remains of which we still find in 

 impressions in shale, slate-clay, and other floetz rocks. 

 Whether the present species, which often resemble these, 

 have arisen from them ; whether the great revolutions on 

 the surface of the earth, which we read in the Book of 

 Nature, contributed to these transitions— we know not. 

 What we know is that from as early a time as the human 

 race has left memorials of its existence upon the earth 

 the separate species of plants have maintained the same 

 properties invariably. 



To be sure, we frequently speak of the transitions and 

 crossings of species; and it can not be denied that some- 

 thing of this kind does occur, though without affecting 

 the idea of species which we have proposed. We must, 

 therefore, understand this difference. 



We perceive the Transitions of a Species, when it loses 

 or changes the properties, which we had considered as 

 invariable in the character. Thus, it would he a transi- 



ment, it began to assume awns, and, when sown in spring, 

 came to maturity during the same summer. 



But this shows only that our idea of the difference 

 between the two kinds of grains had been incorrect; for 

 it is the universal rule, that the character does not con- 

 then, only appear to undergo transitions, when we have 

 considered an organ or a property as invariable which 

 is not so. 



