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 larve of Amphibians, the water passes off 
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 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. 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 parti- 
