50 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



marine Crustacea belonging to two groups, the 

 Trilobites and the Gigantostraca. 



The Arthropods with aerial respiration, descend 

 also, according to the author, either from another 

 branch of the articulated worms, or from a very 

 early bifurcation of the aquatic Arthropod branch. 

 Gegenbaur has endeavoured to show the analogy 

 of the external branchio-tracheas of the larva of the 

 Ephemeridae and the Libellules with the dorsal 

 branchiae of certain Crustacea and Annelids, the 

 formation of internal tracheas being of recent ac- 

 quisition. Be this as it may, the high geological 

 antiquity of the Scorpionidse, which date from the 

 Silurian period, would singularly thrust back the 

 epoch of this hypothetical bifurcation. 



Finally, Haeckel reaches the Vertebrates which 

 are, of all geological groups, those which contribute 

 the most exact documents to the evidence of descent. 

 Here again, individual embryology plays a para- 

 mount part in the demonstration by showing us in 

 all vertebrates the essentially similar evolutionary 

 stages starting from the ovum and preserving a 

 similitude the more continuous the nearer that 

 these groups are to one another in the natural 

 classification. 



The origin of the Vertebrates is made clearer, as 

 Darwin had before pointed out, by the discoveries 

 of Kowalevsky on the unexpected resemblance of 

 the embryology of the Ascidians and of the Am- 

 phioxus, or very low skull-less Vertebrate. The 

 presence of a spinal cord and of a rudimentary 

 marrow in the embryos of the Ascidia enables us 

 to catch a glimpse in the large group of Worms of 



