Page:Popular Science Monthly Volume 16.djvu/484

 This instability of the environment has produced an instability of the flora, and caused those differences which have resulted in the European vegetation of our time.

As before remarked, in speaking of ancient climates, when we go back in time, and particularly Tertiary time, we see the vegetation taking more and more of a tropical character. Hence in these epochs there existed in Europe a multitude of forms which can not live there now. Palms and Cycadeæ (Fig. 2) and large, beautiful ferns were long ago exiled. Other forms, as the laurel, the vine, the ivy, have never quitted the region where they were born, or, at least, where they appeared for the first time.

The number of figures that Saporta has interspersed with his text, representing the principal vegetable types of the past, offer us the still further advantage of comparing species of the same type, and verifying by inspection the respective modifications of these species, and their passage from one to the other. Without doubt, we are far from possessing all the terms of all the series; but what we know of some enables us to judge by analogy that what has happened with one genus may happen with others. See, for example, the forms of Pliocene and Eocene oak (Fig. 7), which show clearly how climate has affected this species from the formation of Gelinden at the base of the Pliocene to the gypsum of Aix, that is, the superior Eocene. The forms represented



here belong to the group of oaks with entire leaves; but there is another group with leaves toothed or lobed, in which we discover analogous modifications. We see that leaves at first oval tend to become more and more slender, and these lanceolate forms express very