3 o6 NATURAL SCIENCE. May, 



of phylogenetic development. No less complete a series is presented 

 by the genealogical tree of the camels, which already appear during 

 the Eocene in North America, spread there in the Miocene and 

 Pliocene, and then first emigrate to the Old World. The pigs also, 

 and the Oreodontidse, Anoplotheriidse, Tragulidae, and the ruminants 

 studied in so masterly a manner by Rutimeyer, afford us more or less 

 complete genealogical series, beside which may be ranged the 

 crocodiles among reptiles, the Amioidei and Physostomi among fishes. 

 If we glance over these phylogenetic series, we observe that the 

 final terms are almost always distinguished from their predecessors 

 by a more pronounced and distinctive differentiation ; and since we 

 are accustomed to assign a higher rank to a specialised organism in 

 which every function is performed by a special arrangement, than to 

 a creature which performs its functions with few and less complicated 

 parts, phylogenetic development, as a rule, implies also progression 

 and perfection. For the existence of the chains of forms already 

 mentioned, to which many others may be added, there is only one 

 rational explanation, namely, that the various links in them have 

 arisen one from another, and are connected by blood-relationship. 

 Moreover, the similarity of the faunas and floras which are nearest to 

 one another in geological age, as well as the geographical distribution 

 of extinct and still existing plants and animals, can only be satis- 

 factorily explained on the assumption of descent. 



But, although an abundance of palaeontological facts can be 

 cited in the most convincing manner in favour of the theory of 

 descent, on the other hand we must not forget that we still know no 

 point of origin for numerous independently arising creatures, and 

 that the connection between the larger divisions of the animal and 

 vegetable kingdoms is by no means so intimate as the theory specially 

 postulates. The jubilation with which the discovery of Avchceoptevyx 

 was greeted at the time shows at best that links had previously been 

 wanting between two classes which among vertebrates undoubtedly 

 exhibited the closest relationships. Further, Avchceoptevyx fills the 

 gap between birds and reptiles only in a very imperfect manner, and 

 affords no indication of the point at which the former have branched 

 from the latter. It may, indeed, be maintained that we find ourselves 

 to-day in greater uncertainty as to the origin of birds than we were 

 twenty-five years ago, when Huxley's brilliant researches on the 

 pelvis of the Dinosauria seemed to have found the bridge between 

 the two classes. Links between the Amphibia and Reptilia are also 

 still wanting. Perhaps they are to be found among the varied 

 Theromorpha, but as yet palaeontology cannot determine the phylo- 

 genetic modification of the amphibian into the reptilian type. No 

 zoologist will deny that the Mammalia hold an entirely isolated 

 position, separated by a wide gulf from birds, reptiles, amphibians, 

 and fishes ; while among all known mammals it is not some old fossil 

 genus, but the duck-bill, still living in Tasmania, which most reminds 



