THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 05 
The evolution of animal life on this earth can clearly, on the whole, be 
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 evidence 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 Paleostracan age, using the term Paleostraca 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 
Paleostraca—arthropod forms which were not crustacean and not arachnid, 
but gave origin both to the modern-day crustaceans and arachnids. The 
history of the recks further shows that these ancient fishes, when they first 
appeared, resembled in a remarkable manner members of the paleostracan 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 Ammoceetes stage of the lamprey, because it was 
formerly considered to be a separate species and received the name of 
Ammocetes. 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 Ammoceetes on the other are 
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