382 DARWINISM chap. 



species), which are named and described in this monogi^aph, and 

 between which, as the authors show, so many connecting links, 

 clearly illustrating the derivation of the newer from the older 

 types, have been detected. On the minds of those who care- 

 fully examine the admirably engraved figures given in the 

 plates accompanying this valuable memoir, or still better, the 

 very large series of specimens from among which the subjects of 

 these figures are selected, and which are now in the museum 

 of the Reichsanstalt of Vienna, but little doubt ^vill, we 

 suspect, remain that the authors have fully made out their 

 case, and have demonstrated that; beyond all controversy, the 

 series Avith highly complicated ornamentation were variously 

 derived by descent — the lines of which are in most cases 

 perfectly clear and obvious — from the simple and unorna- 

 mented Vi^ipara achatinoides of the Congerien-Schichten (the 

 lower diAasion of the series of strata). It is interesting to 

 notice that a large portion of these unquestionably derived 

 forms depart so wddely from the type of the genus Vivipara, 

 that they have been separated on so high an authority as that 

 of Sandberger, as a new genus, under the name of Tulotoma. 

 And hence we are led to the conclusion that a vast number 

 of forms, certainly exhilnting specific distinctions, and accord- 

 ing to some naturalists, differences even entitled to be regarded 

 of generic value, have all a common ancestry." 



It is, as Professor Judd remarks, owing to the exceptionally 

 favom^able circumstances of a long -continued and unbroken 

 series of deposits being formed under physical conditions 

 either identical or very slowly changing, that we owe so com- 

 plete a record of the process of organic change. Usually, 

 some distui'bing elements, such as a sudden change of physical 

 conditions, or the immigTation of new sets of forms from other 

 areas and the consequent retreat or partial extinction of the 

 older fauna, interferes wdth the continuity of organic development, 

 and produces those puzzling discordances so generally met 

 Avith in geological formations of marine origin. "While a case 

 of the kind now described affords e-\adence of the origin of 

 species complete and conclusive, though on a necessarily very 

 limited scale, the very rarity of the conditions which are essential 

 to such completeness serves to explain Avhy it is that in most 

 cases the direct e-vidence of evolution is not to be obtained. 



