HIGH SCHOOL ZOOLOGY. 273 



but it will be understood that although the advance of Mor- 

 phology is constantly improving our evidence from the living 

 forms, and although hitherto undiscovered fossil species are 

 constantly being unearthed, yet the evidence from the palajon- 

 tological side must always be incomplete (§ 8). Phylogenetic 

 classitications must, therefore, be tentative^ except in the case of 

 such modern groups as are distinguished by the profuseness of 

 their fossil remains. The genealogical tree of the Animal 

 Kingdom might be described as buried, with the exception of 

 the terminal green twigs — the existing species ; to trace the 

 connection between these it is necessary to dig down into the 

 sedimentary strata, and to piece together with infinite patience 

 the fragmentary evidence which we there find. It must be 

 remembered, however, that the principle of correlation depending 

 on the constant association of morphological peculiarities (sucli 

 as, for example, a ruminant dentition with a particular confor m 

 ation of the skeleton of the foot), justifies us in making use of 

 evidence from very fragmentary remains. 



12. Such studies may, then, indicate the probable line of de- 

 velopment which has culminated in a particular species ; the dis- 

 cussion of the nature and origin of species will be deferred to a 

 subsequent section. In the meantime, in connection with the 

 foregoing topics the accompanying table may be of use, which 

 gives an approximate estimate of the number of described 

 species in the various classes and orders, and serves to show 

 what groups of animals are in " ascendance" at the present day, 

 as exhibited by their comparative wealth in species, and to call 

 attention to the former wealth as compared with the present 

 poverty of such groups as the Brachiopods, Cephalopods, Crinoids, 

 Pteropods, Trilobites, etc. It makes it apparent, also, how cer- 

 tain highly specialised groups,such as the Teleosts, the Anura, the 

 Passeres, the higher Insecta, preponderate over more primitive 

 allied groups. 



