DEVELOPMENT OF AMMONITES. 213 



spinal cord. This is the permanent state of the lower 

 fishes. In the higher Vertebrata also, the brain at 

 first consists of vesicles, lying one behind the other, 

 which is the persistent form of the lower groups. The 

 embryonic heart of mammals and birds begins in the 

 form of a tube, and subsequently acquires the com- 

 munications between the chambers, which in the 

 reptiles never close. In the Amphibians, the branchial 

 arches really bear gills during the larval state. They 

 are not wanting in the embryos of reptiles, birds, and 

 mammals, any more than the fissures through which, in 

 fish and the larvae of Amphibians, the water passes off 

 after being inhaled. Must we again set forth the only 

 possible explanation of these facts 1 



Before referring to the phenomena which testify the 

 emanation of families from a common root, we will cite 

 one of the most important evidences of recent times, 

 which traces the genesis of species through a great 

 geological period, and exhibits in detail the relations 

 of the development of the individuals to that of the 

 species, genus, and family. We mean L. Wurten- 

 berger's contribution to the geological evidence of the 

 Darwinian theory, to which we have already appealed 

 (p. 97). It relates to the two families of Ammonites, the 

 Planulata and Armata ; of which, according to Wiirten- 

 berger's researches, the latter are developed from the 

 former, as the ribs of the Planulata gradually pass into 

 the spines of the Armata. Of special interest to us are 

 the following passages of the preliminary communication 

 on the discoveries obtained from thousands of specimens, 

 and which will probably not be made public, with all the 

 vouchers, for some years to come. " It gave me parti- 



