OBJECTIONS TO THE THEOKT OF DESCENT. 335 



length of time which has elapsed between the consecutive 

 formations, we may infer that this could nowhere be 

 ascertained. The frequent and great changes in the 

 mineralogical composition of consecutive formations, 

 generally implying great changes in the geography of the 

 surrounding lands, whence the sediment was derived, 

 accord with the belief of vast intervals of time having 

 elapsed between each formation. 



p „^g It is aU-important to remember that natu- 

 ralists have no golden rule by which to dis- 

 tinguish species and varieties ; they grant some little 

 variability to each species, but, when they meet with a 

 somewhat greater amount of difference between any 

 two forms, they rank both as species, unless they are 

 enabled to connect them together by the closest inter- 

 mediate gradations ; and this, from the reasons just as- 

 signed, we can seldom hope to effect in any one geological 

 section. Supposing B and C to be two species, and a 

 third, A, to be found in an older and underlying bed ; 

 even if A were strictly intermediate between B and C, 

 it would simply be ranked as a third and distinct species, 

 unless at the same time it could be closely connected by 

 intermediate varieties with either one or both forms. 

 Ifor should it be forgotten, as before explained, that A 

 might be the actual progenitor of B and C, and yet would 

 not necessarily be strictly intermediate between them in 

 all respects. So that we might obtain the parent-species 

 and its several modified descendants from the lower and 

 upper beds of the same formation, and, unless we obtained 

 numerous transitional gradations, we should not recog- 

 nize their blood-relationship, and should consequently 

 rank them as distinct species. 



