580 ZOOLOGY SECT. 



Mammals also appear to have had a reptilian origin; the 

 numerous reptilian characters of the Monotremata certainly point 

 in this direction, and the reproductive processes of that group and 

 of the Marsupials help us to understand the stages by which the 

 large-yolked egg of the ancestral form, developed outside the body, 

 gave place to the secondarily alecithal egg of the typical Mammal, 

 developed within the uterus, and nourished by a complete 

 placenta. 



The following diagram may serve as a rough illustration of the 

 views set forth in the preceding pages 



MAMMALIA 



AMPHIBIA 

 TELEOSTOMI \ 



\l DIPNOI 



/HOLOCEPHALI 

 Y / EXISTING 

 / '/' ELASMOBRANCHII 



PRIMITIVE ELASMOBRANCHII 



CYCLOSTOMATA 



OSTRACODERMI 



ACRANIA 



*, 



UROCHORDA 

 ADELOCHORDA 



FIG. 1168. Diagram illustrating the Mutual Relationships of the Phyla of Animals. 



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 case, been discussed. 



It cannot be too strongly insisted upon 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 the majority of cases the only evidence is 

 to be sought in anatomy and embryology. 



