THE SUCCESSION OF ANIMAL FORMS 179 



origin, come into being simultaneously everywhere, we shall 

 arrive at one of the laws of creation, and one probably con- 

 nected with the gradual change of the physical conditions of 

 the world. 



Another general truth, obvious from the facts which have 

 been already collected, is the periodicity of introduction of 

 species. They come in by bursts or flood tides at particular 

 points of time, while these great life waves are followed and 

 preceded by times of ebb in which little that is new is being 

 produced. We labour in our investigation of this matter 

 under the disadvantage that the modern period is evidently 

 one of the times of pause in the creative work. Had our time 

 been that of the early Tertiary or early Mesozoic, our views as 

 to the question of origin of species might have been very dif- 

 ferent. It is a striking fact, in illustration of this, that since 

 the glacial age no new species of mammal, except, possibly, man 

 himself, can be proved to have originated on our continents, 

 while a great number of large and conspicuous forms have 

 disappeared. It is possible that the proximate or secondary 

 causes of the ebb and 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 of nature. 



It results from the above and the immediately preceding 

 statement, that specific and generic types enter on the stage in 

 great force, and gradually taper off towards extinction. They 

 should so appear in the geological diagrams made to illustrate 

 the succession of living beings. This applies even to those 

 forms of life which come in with fewest species and under the 

 most humble guise. What a remarkable swarming, for ex- 

 ample, there must have been of Marsupial Mammals in the: 

 early Mesozoic, and in the Coal formation the only known 

 Pulmonate snails, five or six in number, belong to four generic 



s. E. 13 



