THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 65 



The evolution of animal life on this earth can clearly, on the whole, he 

 described as a process of upward progress culminating in the highest form — 

 man ; but it must always be remembered that whole groups of animals such as the 

 Tunicata have been able to survive owing to a reverse process of degeneration. 



If there is one organ more than another which increases in complexity as 

 evolution proceeds, which is the most essential organ for upward progress, surely 

 it is the central nervous system, especially that portion of it called the brain. 

 This consideration points directly to the origin of vertebrates from the most 

 highly organized invertebrate group — the Arthropoda — for among all the groups 

 of animals living on the earth in the present day they alone possess a central 

 nervous system closely comparable with that of vertebrates. Not only has an 

 upward progress taken place in animals as a whole, but also in the tissues them- 

 selves a similar evolution is apparent, and the evidence shows that the vertebrate 

 tissues resemble more closely those of the arthropod than of any other inverte- 

 brate group. 



The evi deuce of geology points to the same conclusion, for the evidence of 

 the rocks shows that before the highest mammal — man— appeared, the dominant 

 race was the mammalian quadruped, from whom the highest mammal of all — 

 man — sprung ; then comes, in Mesozoic times, the age of reptiles which were 

 dominant when the mammal arose from them. Preceding this era we find in 

 Carboniferous times that the amphibian was dominant, and from them the next 

 higher group — the reptiles — arose. Below the Carboniferous come the Devonian 

 strata with their evidence of the dominance of the fish, from whom the 

 amphibian was directly evolved. The evidence is so clear that each succeeding 

 higher form of vertebrate arose from the highest stage reached at the time, 

 as to compel one to the conclusion that the fishes arose from the race which 

 was dominant at the time when the fishes first appeared. This brings us to the 

 Silurian age, in which the evidence of the rocks points unmistakably to the sea- 

 scorpions, king-crabs, and trilobites as being the dominant race. It was preceded 

 by the great trilobite age, and the whole period, from the first appearance of the 

 trilobite to the time of dwindling away of the sea-scorpions, may be designated 

 the Pakeostracan age, using the term Palaeostraca to include both trilobites and 

 the higher scorpion and king-crab forms evolved from them. The evidence of 

 geology then points directly and strongly to the origin of vertebrates from the 

 Palaeostraca — arthropod forms which were not crustacean and not arachnid, 

 but gave origin both to the modern-day crustaceans and arachnids. The 

 history of the rocks further shows that these ancient fishes, when they first 

 appeared, resembled in a remarkable manner members of the palaeostracan group, 

 so that again and again paleontologists have found great difficulty in determin- 

 ing whether a fossil is a fish or an arthropod. Fortunately, there is still alive 

 on the earth one member of this remarkable group— the Limulus, or King- 

 Crab. On the vertebrate side the lowest non-degenerate vertebrate is the 

 lamprey, or Petromyzon, which spends a large portion of its existence in a 

 larval stage, known as the Ammoccetes stage of the lamprey, because it was 

 formerly considered to be a separate species and received the name of 

 Ammoccetes. The larval stages of any animal are most valuable for the study 

 of ancestral problems, so that it is most fortunate for the solution of the ancestry 

 of vertebrates that Limulus on the one side and Ammoccetes on the other are 



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