REVIEW OF THE HISTORY OF LIFE. 257 



life emerge at one time in all parts of the world. Both cause, 

 may be mfluential, as the one does not exclude the other and 

 here ,s reason to believe that both are natural facts. Should 

 .t be Ml .matcly proved that species allied and representative, 

 but drstmct m ong.n, come into being simultaneously every! 

 where we shall arnve at one of the laws of creation, and one 

 probably connected with the gradual change of the physica' 

 conditions of the world. P"yi>icai 



snede^'T/""'"^ "■""'.'' ""^ Periodicity of introduction of 

 Ssof , ^^";T u ^^^""'' or (lood-ti<Ies at particular 

 points of ,me while these great life-waves are followed and 

 preceded by t.mes of ebb in which little that is new is being 

 produced We labour in our investigation of this matter tmTer 

 he disadvantage that the modern period is evidently one of 



hnt TX ^TV" •"'' "'^''™ """•'^' "'iJ o^r time been 

 that of he early I erfary or early Mesozoic, our views as to 



the question of origin of species might have been very differ- 

 ent, t IS a striking fact, in illustration of this, that since the 

 Glacial age no new species of mammal can be proved to have 

 originated on our continents, while a great number of large 

 and conspicuous forms have disappeared. It is possible that 

 he proximate or secondary causes of the ebb an.l flow of 

 life-production may be in part at least physical; but 

 other and more important efficient causes may be behind 

 these. In any case these undulations in the history of life 

 are in harmony with much that we see in other departments 

 oi nature. '■ 



It results from the above and the immediately preceding 

 statement that specific and generic types enter on the stage 

 in great force, and graduiilly taper off toward extinction. They 

 should so appear in the geological diagrams made to illustrate 

 the succession of life. This applies even to those forms of life 

 which corne in with fewest- species and under the most humble 

 gu.se What a remarkable swarming, for example, there must 

 have been of Marsupial Mammals in the early Mesozoic ; and 



s 



