by Deciduous Plants in the Tertiary Floras. 291 



all the floras, starting from the Upper Eocene, that we meet with 

 European forms (limited in number, it is true, but with remark- 

 able fixity), we are compelled to see in them, not the result of 

 an accident of locality, but one of the elements of the vegetation 

 of that period — an element which must be taken into considera- 

 tion in analyzing the totality from which it depends. The 

 regular development of this same element, at first very slowly, 

 constitutes the most salient feature of the Tertiary vegetation in 

 its course towards modern times. We have not here to appre- 

 ciate this progress, but to seize the true character of this group 

 of species at its origin, when, far from predominating, it is, so 

 to speak, lost in the midst of the most varied exotic forms. As 

 they remove from their starting-point, the organisms {essences) 

 with deciduous leaves tend progressivelyto become what theyare at 

 present ; but, notwithstanding the chain which binds their present 

 to their former state, it does not necessarily follow that their mode 

 of being was the same at all times. It would be to draw a forced 

 conclusion from what they are under our eyes if we pass beyond 

 a simple analogy of form. The mere fact of their association 

 with plants the presence and preponderance of which announce 

 an order of things different from that which exists in our days 

 is an important indication that these species were far from being 

 then adapted to the external conditions to which their congeners 

 are now subjected, and consequently that some difference must 

 distinguish them from the similar organisms of the present day. 

 The existence of an annual temperature attaining an average 

 of 68°-77° F. (20°-25° Cent.) at the time of the gypsum of Aix 

 follows from all the indications furnished by the plants of the 

 period. Nor is the successive diminution of the temperature 

 less evident from the gradual disappearance of all the tropical 

 forms — a disappearance which need not have taken place if these 

 forms had originally been adapted to a ruder climate than that 

 which is now necessary to them. In fact, if there is nothing in 

 opposition to the assumption that the types which have since 

 continued to be European were at first adapted to a hotter climate, 

 the contrary supposition (that is to say, that of tropical types 

 conformed to a colder climate) appears to be by no means admis- 

 sible, not only because these types, from their organization, do 

 not appear to be susceptible of such a deviation, but also because 

 (leaving the possibility of this out of the question) their associa- 

 tion, their mode of grouping, their preponderance, and their 

 analogy with the most characteristic forms of tropical regions 

 sufficiently indicate that the general vegetation of this epoch is 

 the expression of a temperature sufficiently high, or at least 

 sufficiently uniform, to give rise to the external conditions which 

 are now characteristic of the countries near the tropics. 



