VARIATION IN TIME 159 



in the great group of Ammonites, the Lytocerata, 

 the Arietitce, the Perisphinctce, and the Hoplites, to 

 mention only the most typical. The polyphyletic 

 genera are also very common in the other groups 

 of Invertebrates : the Spirifers, the Rynchonellse, 

 the Terebratulae, among Brachiopods ; the Oysters, 

 the Pectines, the Trigonise, the Pholadomyse, the 

 Hippuritse, among the Lamellibranchs ; the Pleuro- 

 tomaries, the Trochi, the Paludines, the Turritella, 

 the Nerinese, the Cerithia, the Nassse, the Pleuro- 

 tomas, the Murices, the Coni, among the Gastropods ; 

 and the Orthocerata, the Nautiluses, among the tetra- 

 branchial Cephalopods, are examples very familiar 

 to all palaeontologists. I shall show further on, 

 that polyphyletism is also very evident in a certain 

 number of the genera of Tertiary Mammals. It 

 can already be foreseen that all, or nearly all, 

 the genera of fossil animals when they have been 

 studied with precision in their mutations, will 

 become more or less polyphyletic. Monophyletism 

 is hardly noticed except in genera poor in species, 

 apparently endowed with a vitality of very little 

 energy, and showing a weak tendency to variation 

 in time. Perhaps even this monophyletism is 

 only seeming and provisional, and is limited to 

 certain periods and to certain regions for the same 

 type, and is, no doubt, subject to intermittences in 

 the expansive force of the branch. 



DISCONTINUOUS SERIES. The essential criterion 

 of a phyletic series is continuity. There exist, 

 however, a pretty good number of indisputably 

 natural branches in which this continuity is in- 

 terrupted by gaps, that is to say, by the absence 



