No. 3.] MODE OF EVOLUTION IN THE MAMMALIA. 365 



humerus, but in this case the bone is of a more primitive type, 

 with single bicipital groove and no bicipital tubercle, or only a 

 very rudimentary one. 



The steps of modernization which may be observed in fol- 

 lowing out the history of many different groups of mammals 

 are seen to keep curiously parallel, as may be noticed, for 

 example, in the series of skulls figured by Kowalevsky (No. 

 26, PL IX.), where we find similar changes occurring in such 

 families as the pigs, deer, antelopes, horses, elephants, etc. 

 Indeed, one may speak with propriety of a Puerco, or Wasatch, 

 or White River type of skull, which will be found exempli- 

 fied in widely separated orders. Of course there are excep- 

 tions, some types being in advance of, and others lagging 

 behind, the standard of their age, and others, again, attaining 

 great peculiarities of structure which will not be perpetuated ; 

 but, on the whole, the type of skull or limbs keeps strikingly 

 true to the standard. These facts emphasize the truth of the 

 proposition laid down by Cope : " From these and many analo- 

 gous cases, the general law may be deduced, that identical modi- 

 fications of structure, constituting evolution of types, have 

 supervened on distinct lines of descent " (No. 8, p. 343). Of 

 similar import are the simultaneous and similar variations of 

 different orders, according to geographical location, such as the 

 characters of the Lepidoptera in the different Malay islands, 

 to which Wallace has directed attention (No. 56), and which 

 is exhibited in such a striking way by the coloring of Arctic 

 and desert animals, and very many other well-known phenom- 

 ena. All this but illustrates anew the conclusion long since 

 universally accepted, that no natural system of classification can 

 be founded upon the presence or absence of a single character. 



Parallelism may, however, extend much further than this, and 

 embrace, not single characters only, but whole series of them. 

 It is this fact which renders the proper reference of such forms 

 as Leptomeryx so very difficult. If the interpretation of the 

 tylopodan phylogeny which was given in my former paper be 

 correct, then the camels and the true ruminants have no common 

 ancestor nearer than the Dichobunidoe, animals with quinque- 

 tuberculate upper molars, complete dentition, short limbs, unre- 

 duced ulna and fibula, and tetradactyl feet, all the digits being 

 free. The two series have independently acquired tetra-selen- 



