THE GEOLOGICAL KECOKD. 107 



an indefinitely long time. Change is no exact measure 

 of variation. 



It is sometimes objected that phylogenies are insecure 

 speculations, that they are made up of series put to- 

 gether according to preconceived theory, that the con- 

 clusions have often proved erroneous.. As the gaps 

 become more completely filled up, certain forms are 

 found to have been placed in the wrong order or to 

 belong to diverging branches rather than to the same 

 stem. But there can scarcely be any doubt that in 

 the main these phylogenies represent the real course 

 of evolution, and such mistakes in details do not 

 shake our faith in the correctness of the conclusions 

 as a whole. > 



Very important also is the evidence of palaeontology, 

 concerning the gradual character of the transition from 

 one form to the other. We need not describe in detail 

 the case of the horse, which is familiar to all, but shall 

 only mention that it can be traced from an unspecialised 

 comparatively small Eocene ancestral mammal, with five 

 digits on each foot, a normal short skull, and the full 

 complement of short rooted teeth. The complex pattern 

 on the grinding surface of the teeth can be seen to evolve 

 by almost insensible steps from the original six-cusped 

 form, just as the lateral digits gradually become reduced 

 on the feet. Exactly how gradual these transitions have 

 been we cannot often yet say, but the more complete 

 the evidence the smaller appear to have been the steps. 

 Wonderfully gradual are the transitions described by 

 Beecher in Trilobites and Brachiopods, by Hyatt and 

 others in Ammonites, and in numberless other cases. 

 Neumayer has traced the gradual evolution of Paludiua 

 neumayri, found in the lowest Pliocene deposits of 

 Slavonia, into P. Hoernesi of the uppermost layer*. 

 Hoernes has shown that the Pliocene Cancellaria cancel- 

 lata is intermediate between the Miocene and the recent 

 forms of the same species, while Hilgendorf has followed 



