DEVELOPMENT OF AMMONITES. 



213 



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 ac- 

 quires the communications between the chambers, 

 which in the reptiles never close. In the Amphibians, 

 the branchial arches really bear gills during the lar- 

 val 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 ofif after being inhaled. 

 Must we again set forth the only possible explanation 

 of these facts? 



Before referring to the phenomena which testify the 

 emanation of families from a common root, we will cite 

 one of the rnost 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. Wiirten- 

 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 particu- 

 lar pleasure," says Wiirtenberger, " when, after divers 



