20 THE ORIGIN OF VERTEBRATES 
to unstriped muscle, and thence to the highest forms of striated 
muscle; of the nervous connecting strands, from undifferentiated to 
fine strands, then to thicker, more separated ones, resembling non- 
medullated fibres, and finally to well-differentiated separate fibres, 
each enclosed in a medullated sheath. 
In the connective tissue group, bone is confined to the vertebrates, 
cartilage is found among invertebrates, and the closest resemblance 
to vertebrate embryonic or parenchymatous cartilage is found in the 
cartilage of Limulus. Also, as Gegenbaur has pointed out, Limulus, 
more than any other invertebrate, possesses a fibrous connective 
tissue resembling that of vertebrates. 
In the muscular group, Biedermann, who has made a special 
study of the physiology of striated muscle, says that among inver- 
tebrates the striated muscle of the arthropod group resembles most 
closely that of the vertebrate. 
In the nervous group the resemblance between the nerve-fibres 
of Limulus and Ammoccetes, both of which are devoid of any marked 
medullary sheath, is very apparent, and Retzius points out that the 
only evidence of medullation, so characteristic of the vertebrates, is 
found in a species of prawn (Palemon). In all these cases the 
nearest resemblance to the vertebrate tissues is to be found in the 
arthropod. 
THE EVIDENCE OF PALAZONTOLOGY. 
Perhaps the most important of all the clues likely to help in the 
solution of the origin of vertebrates is that afforded by Geology, for 
although the geological record is admittedly so imperfect that we 
can never hope by its means alone to link together the animals at 
present in existence, yet it does undoubtedly point to a sequence in 
the evolution of animal forms, and gives valuable information as to 
the nature of such sequence. In different groups of animals there 
are times when the group can be spoken of as having attained its 
most flourishing period. During these geological epochs the dis- 
tribution of the group was universal, the numbers were very great, 
the number of species was at the maximum, and some of them had 
attained a maximal size. Such races were at that time dominant, 
and the struggle for existence was essentially among members of the 
same group. At the present time the dominant race is man, and the 
