The Principles of Palaeontology. 151 



Permo-carboniferous age, and have demonstrated that the evolu- 

 tion of the Ammonite type must have taken place in the southern 

 and eastern Mediterranean zone. 



Whilst acknowledging the importance of the new results with 

 which Palaeontology has enriched the history of the evolution of 

 organisms, it must be allowed that this science so far has not 

 done air that was expected of it, especially as to what concerns 

 the origin of the great subdivisions of the animal kingdom. Thus 

 the Brachiopods, Insects and Mammals appeir isolated, notwith- 

 standiDg that their remains have been found in more and more 

 ancient deposits; the representatives of the ancestral forms of 

 these groups thus far have not appeared. Frequently some especial 

 cause for each particular case can be assigned for these gaps. 

 Thus, for instance, the ancestors of the Yertebrates were proba- 

 bly soft animals, as seems' to be proved by the existence of 

 Amphioxus ; naturally they would not leave any traces in the 

 rocks. The same may be said of the progenitors of the Batrach- 

 ians, which were cartilaginous. Or, again, the group in its 

 entirety is not aquatic, and leaves but few representatives, as is 

 the case with birds and insects. Lastly, it may happen that the 

 hard parts which are of much importance in affording us an 

 acquaintance with a large number of individual fossils, still do 

 not permit of any precise determination of anatomical details, 

 as in the case of the Crustacea, the Gasteropoda, and many of 

 the Coelenterata. 



Saltation.* — The insuflSciency of materials, so often invoked, 

 partly explains then the gaps observed, and weakens the im- 

 portance of the arguments deduced from those against evolution. 

 Nevertheless, this idea is not sufficient ; it does not explain, for 

 example, why the Acephala are never found in the Cambrian,f 

 while the Gasteropoda are numerous there ; and why they appear 

 suddenly in the middle Silurian in various forms and bearing all 

 the essential characteristics of a group. [N'either does it explain 

 why, in more recent epochs, important gaps exist between fami- 

 lies in groups whose representative fossils are very numerous 

 and well preserved. If we examine the succession of life in 

 time, or study the contemporaneous forms of any given epoch, 



♦ Eimer, Entstehung der Arten, Jena, 1887. 



I- Later studies of these faunas by Walcott and others, show the presence of Acephala In the 

 lower Cambrian.— Ed. 



