THE ARGUMENTS FROM DISTRIBUTION. 487 



quiry we saw to be, that while a few modern higher types yield 

 signs of having been developed from ancient lower types; 

 and that while there are many modern types which may have 

 been thus developed, though we are without evidence that 

 they have been so ; yet that " any admissible hypothesis of 

 progressive modification must be compatible with persistence 

 without progression through indefinite periods." Now these 

 results are quite congruous with the hypothesis of evolution. 

 As rationally interpreted, evolution must in all cases be under- 

 stood to result, directly or indirectly, from the incidence of 

 forces. If there are no changes of conditions entailing organic 

 changes, organic changes are not to be expected. Only in 

 organisms which fall under conditions leading to additional 

 modifications answering to additional needs, will there be 

 that increased heterogeneity which characterizes higher forms. 

 Hence, though the facts of palaeontology cannot be held con- 

 clusive proof of evolution, yet they are congruous with it; 

 and some of them yield it strong support. 



§ 141. One general truth respecting distribution in Time, 

 is profoundly significant. If, instead of contemplating the 

 relations among past forms of life taken by themselves, we 

 contemplate the relations between them and the forms now 

 existing, we find a connexion which is in harmony with the 

 belief in evolution but irreconcilable with any other belief. 



Xote, first, how full of meaning is the close kinship 

 existing between the aggregate of organisms now living, and 

 the aggregate of organisms which lived in the most recent 

 geologic times. In the last-formed strata, nearly all the 

 imbedded remains are those of species which still flourish. 

 Strata a little older contain a few fossils of species now ex- 

 tinct, though, usually, species greatly resembling extant ones. 

 Of the remains found in strata of still earlier date, the ex- 

 tinct species form a larger percentage ; and the differences be- 

 tween them and the allied species now living are more marked. 



That is to say, the gradual change of organic types in Time, 

 33 



