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 Eetzius points out that the 

 only evidence of medullation, so characteristic of the vertebrates, is 

 found in a species of prawn (Palamion). In all these cases the 

 nearest resemblance to the vertebrate tissues is to be found in the 

 arthropod. 



The Evidence of Paleontology. 



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 



