108 T7ie American Naturalist. [February, 



been asserted with good reason to have descended from the 

 former. It has been shown, however, that the Equus caballus 

 could not have descended from the European Hippotheriwm 

 tnediterraneuni, and hence some writers have jumped to the 

 conclusion that the alleged phyletic relation of the two genera 

 does not exist, The reasons for denying this descent are, 

 however, presented by specific characters only, and the generic 

 characters are in no way affected. Further, we know several 

 species of Hippotherium which could have given origin to the 

 Equus caballus probably through intermediate species of 

 Equus. 



Some naturalists are very uncritical in criticising phylo- 

 genies in the manner I have just described. They often ne- 

 glect to ascertain the definitions given by an author to a 

 group alleged by him to be ancestral ; but fitting to it some 

 definition of their own, proceed to state that the ancestral posi- 

 tion assigned to it cannot be correct, and to propose some new 

 division to take its place. It is necessary to examine, in such 

 cases, whether the new group so proposed is not really in- 

 cluded in the definition of the old one which is discarded. 



The fact that existing genera, families, etc., are contem- 

 porary need not invalidate their phyletic relation. Group 

 No. 1 must have been contemporary with group No. 2, at the 

 time that it gave origin to the latter, and frequently, though 

 always, a certain number of representatives of group No. 1 

 have not changed, but have persisted to later periods. Some 

 genera, as, e. g., Crocodilus, have given origin to other genera 

 (i. e., Diplocynodon) and have outlasted it, for the latter genus 

 is now extinct. The lung fishes, Ceratodus, are probably an- 

 cestral to the Lepidosirens, but both exist to-day. Series of 

 genera, clearly phyletic, of Batrachia Salientia, are contem- 

 poraries. Of course we expect that the paleontologic record 

 will show that their appearance in time has been successive. 

 But many ancestors are living at the same modern period as 

 their descendents, though not always in the same geographic 



