DESCRIPTION" OF SPECIES. PP.109 



tion of marine mollusks, or vertebrates. Modern researcli 

 shows that many single forms continue to live after the 

 period of their culmination. We must then consider the 

 question of the culmination and decadence of species. The 

 evidence from the existence of exceptionally long lived 

 forms, in any series of strata, must be considered of slight 

 value. 



Again, in assigning their relative value to the component 

 parts of any of these three classes of organisms, we must 

 consider that representative or closely allied forms should 

 have hardly less value than identical ones, in certain cases. 

 This is especially true where we find many closely related, 

 and few identical species. We must not necessarily con- 

 clude that the age of two formations in such a case is dif- 

 ferent, but that surrounding conditions had sufficient power 

 to modify specific characters. We must assign consider- 

 able value to resemblance, or difference, in type, for a 

 change of type implies a change in the conditions of ex- 

 istence. 



It is good evidence that we have to deal with a more re- 

 cent formation, when we find it to show a decadence of old 

 forms, and an introduction of new ones, destined to reach 

 their culmination at a later period. Thus if we find, in a 

 series of rocks, plants characteristic of the Carboniferous 

 formation, and perceive that these die out and disappear, 

 we should not conclude from their mere presence, that the 

 age of the strata is Carboniferous, but rather that it is Per- 

 mian. So also the finding of genera and species, even 

 identical with those of the Trias or Jurassic, would not 

 necessarily imph^ a Triassic or Jurassic age. If we find 

 them to be exceedingly rare, their presence is rather indi- 

 cative of a formation older than the Trias or the Jurassic. 



It is only by taking into consideration all the above named 

 characters, and other points which may be presented by 

 the entire body of specimens, that we can determine the 

 nature of the evidence offered by the life of a formation. 

 It will not suffice to say arbitrarily, that this or that fea- 

 ture is without value as evidence. Circumstances might 

 reverse the normal relative weight of the evidence from the 



