156 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



impartial observer. For these series to be demon- 

 strative, it is plain that there must be no lacunce in 

 them, or that such lacunce should be so infrequent as 

 not to interrupt the general view of the continuity 

 of the variation. Each of the closely linked terms 

 of any series has received from Waagen the 

 name of mutation, a term which it would be most 

 advantageous to use more frequently than we do 

 in palaeontological nomenclature by designating 

 as ascending mutations the forms which follow each 

 other while rising to our present period, and descend- 

 ing mutations those met with when sjnking into 

 ever earlier strata. It thus becomes possible 

 to recognize the intensity of the chronological 

 variation, that is to say, the action of time 

 on the characteristics and structure of one zoo- 

 logical type. And I purposely employ in this first 

 sketch the rather vague term of type instead of the 

 terms species or genus, because we shall have to 

 discuss later on how these two degrees in the zoo- 

 logical hierarchy may be introduced between the 

 different terms of the same phyletic and well- 

 arranged series. 



The first series of forms well studied and solidly 

 established were first discovered in the world of 

 Invertebrates. Waagen made us acquainted with 

 the series of mutations of a group of Jurassic Am- 

 monites, that of the Ammonites subradiatus. Neu- 

 mayr, extending this fertile idea, illustrated it 

 with the magnificent example furnished by the 

 series of rapid evolutionary forms of the Pliocene 

 Paludines of the Danube basin. I must refer the 

 reader to the summary indications already given 



