278 TEE TRANSFORMATIONS OF THE ANIMAL WORLD 



of a slighter variation, during which the branch 

 pursues with deliberation and regularity the normal 

 course of its development. It is thus that the 

 Ammonites of the genera Neumayria manifest 

 at the Kimmeridgian epoch in the limestones of 

 Crussol a veritable explosion of manifold forms 

 contrasting with the dearth of variations of the 

 same branch during the Oxfordian and the Se- 

 quanian. We may also quote in the same group of 

 Ammonites the brilliant blossoming, at the Barre- 

 mian epoch, of the Pulchelliidae, of which there are 

 only found, for the first time, a few meagre repre- 

 sentatives in the Hauterivian ' and Valanginian 

 stages which precede it. I may also mention, in 

 the same order of ideas, the fine expansion at the 

 Miocene epoch, of the Urchins of the Clypeaster 

 genera and of the Molluscs of the family of the 

 Pectinidae, both of them groups which are very 

 poor in species, and, moreover, of small dimensions, 

 in the first half of Tertiary times. The Verte- 

 brates also present analogous facts : the abrupt 

 expansion of the Ichthyosaurs in the Lias, of the 

 Pythonomorphs in the White Chalk, of the Sauropod 

 Dinosaurs in the upper Jurassic, the multiplicity 

 of the branches of the Lophiodons in the middle 

 Eocene, of the Palceotheria in the upper Eocene, of 

 the Antelopes in the upper Miocene, of the Cervidse 

 in the recent Pliocene, etc., indicate, in these differ- 

 ent groups moments of very intense vitality which 

 agree well enough with the hypothesis of a more or 

 less sudden divergence of their numerous branches. 

 Thus the evolution of fossil beings appears to 

 present two distinct mechanisms : the one contin- 



