596 ZOOLOGY SECT. 



the uterus, and nourished by a complete placenta. Though 

 paleontology does not reveal to us the actual reptilian progenitors 

 of the Mammalia, yet, as already pointed out, there are some of the 

 Anomodontia (Theriodonta) which have pronounced mammalian 

 resemblances in skeleton and teeth. 



The following diagram may serve as a rough illustration of the 

 view set forth in the preceding pages : 



MAMMALIA 



AVES 



AMPHIBIA 

 TELEOSTOMI \ i 



\ DIPNOI 



1/HOLOCEPHALI 

 / / EXISTING 

 l^^ ELASMOBRANCHII 



PRIMITIVE ELASMOBRANCHII 



CYCLOSTOMATA 



OSTRACODERMI 



ACRANIA 



x, 

 "- 



UROCHORDA 

 HEMICHORDA 



FIG. 1249. Diagram illustrating the Mutual Relationships of the Chordata. 



ON THE MUTUAL RELATIONS OF THE PHYLA OF ANIMALS. 



It will be advantageous in concluding our survey of the animal 

 kingdom to sum up with a few remarks as to the phylogeny 

 of the primary groups or phyla, since that of the sub-divisions 

 of each phylum has already, in nearly every instance, been 

 discussed. 



It cannot be too strongly emphasised that in the majority of 

 cases it is useless to seek for the ancestors of any animal among 

 existing forms. As far as we know, most living species are culmina- 

 tions terminal branches of the great tree, not leading directly to 

 any other form, but connected only at the fork of a branch. It is, 

 as a rule, only among fossils that actual ancestral forms are to 

 be looked for ; hence the area of strict phylogenetic investigation 

 is very limited, and in most instances the only evidence is to be 

 sought in anatomy and embryology. 



Not only are most existing species culminations, and therefor- 



