166 Report of the State Geologist. 



The genus TF^Zmz^cA^, in particular, with its two peculiar large 

 leaves, is one of the most curious types of the vegetable kingdom. 

 From the Darwinian point of view, these facts, provided they 

 are general enough to claim consideration, are capable of an easy 

 explanation. It is known that the intermediate types *in general 

 disappear rapidly in cases where the evolution determines a 

 marked superiority of the new types over those from which they 

 are derived. These latter may, however, subsist, provided the 

 difference between them and the new forms is sufficiently great, 

 so that the vital concurrence is not too unfavorable for them, that 

 is to say, provided the evolution takes place so rapidly that the 

 new forms soon become distinct. As to the intermediate forms, 

 placed, so to say, between two destructive causes, they must, in 

 order to maintain themselves, undergo a special evolution, in a 

 sense peculiar to themselves, and that will occur only if variations 

 appear in an organ which has - not been already affected by the 

 evolution of the principal type. Thus protected, so to say, they 

 may persist during long periods without any important modifica- 

 tions ; this is the case with all the types we have just cited. 

 Such modifications will the more readily occur, as we have seen, 

 as the evolution is more accelerated during the periods when the 

 new types are in process of formation. 



§ 4. General Tendency or Evolution. 



Hypothesis of the vital force in different groups. — We have 



already planned to pursue still farther this philosophic synthesis 

 of the phenomena of evolution. The meehanism of Evolution is 

 considered sufficiently known to justify us in turning attention 

 to the determination of its general significance. 



One of the most interesting hypotheses which has been pro- 

 posed is that which considers the various groups, such as the 

 species, genus, family, as having a peculiar individuality, and as 

 presenting the same vital phenomena as do single individuals. 

 A given group must then, according to this theory, necessarily 

 come into existence, grow, reach a climax, decrease and finally 

 die, after having in some cases reproduced themselves in some 

 way, by giving origin to other groups of the same value and a 

 little different, so perpetuating the form with a slight modifica- 

 tion. This ingenious hypothesis would explain why, with no 



