of Geological Periods. 269 



analogical conclusions derived from the study of this plant will 

 have the more probability in proportion as the affinity is move 

 evident and the genus of which it forms part has more exclusive 

 adaptations and better-defined habits. 



One condition, however, is necessary to make sure of reach- 

 ing the truth by this course — namely, that the generic affinity of 

 the ancient species be supported by actual proofs, or, at least, 

 that there be very little uncertainty about it. By adopting too 

 readily a multitude of determinations indicated by palaeonto- 

 logical botanists during the last few years, we should inevitably 

 be exposed to the risk of building a superstructure Avithout firm 

 bases. 



There is another important remark, of which, indeed, we 

 shall have to make immediate application — namely, that certain 

 determinations, although apparently doubtful, are in reality 

 much less so. If, instead of being considered isolatedly, they 

 relate to genera of which the repeated and successive presence 

 is so well established that the fact of their ancient existence 

 cannot be made a matter of doubt, there is then established 

 between the various members of a series of graduated stages 

 a sort of solidarity which causes the eyes and the mind to 

 habitually recognize without hesitation impressions the true 

 nature of which would escape them under other circumstances. 

 The smallest indications are sometimes sufficient to reveal this 

 to practised eyes. It is, moreover, certain that the types which 

 continue characteristic of a region reappear almost always with 

 great persistency in the fossil state in the strata of that region. 

 This is one of the most settled general phenomena that can be 

 appealed to in geology, not only in the world of plants, but 

 also in that of animals of all classes. It is the case with the 

 ancient European flora ; and the types of vegetation which now 

 characterize exclusively tlie northern temperate zone in the two 

 hemispheres are reproduced in the fossil state, not only in the 

 period immediately preceding our present one, but also through 

 a long series of stages, until we reach the point where, dis- 

 coveries being deficient, the investigation of these types is of 

 necessity interrupted. These types are not the only ones that 

 wc can determine with certainty and with advantage to the 

 question before us. Without speaking of those which have de- 

 creased in Europe, or which have since abandoned this continent 

 to maintain themselves elsewhere, there exist others which arc 

 now observed only in the vicinity of the tropics. The more the 

 affinities of the species of this category can be rigidly defined, 

 the more possible is it to fix the probable degree of temperature 

 which their presence in ancient Europe must lead us to accept 

 for the period at which they lived. Here I find a difficulty 



