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 invertebrates 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 Ammocoetes, 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 (_Palæmon_). In all these cases the nearest resemblance to the vertebrate tissues is to be found in the arthropod. THE EVIDENCE OF PALÆONTOLOGY. 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 distribution 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 {21}struggle for existence is essentially between the members of that race, and not between them and any inferior race. The effect of such conditions is, as Darwin has pointed out, to cause great variation in that group; in consequence of that variation and that dominance the evolution of the next higher group is brought about from some member of the dominant group. Thus the present age is the outcome of the Tertiary period, a time when giant mammals roamed the earth and left as their successors the mammals of the present day; a time of dominance of quadruped mammals; a time of which the period of maximum development is long past, and we now see how the dominance of the biped mammal, man, is accompanied by the rapid diminution and approaching extermination of the larger mammals. No question can possibly arise as to the immediate ancestor of the biped mammal; he undoubtedly arose from one of the dominant quadrupedal mammals.