18 THE ORIGIN OF VERTEBRATES 
pass from the sponges to the arthropods—a progress which is mani- 
fested, first by the concentration of nervous material to form a central 
nervous system, and then by the increase in substance and complexity 
of that nervous system to form a higher and a higher type, until the 
culmination is reached in the nervous system of the scorpions and 
spiders. No upward progress is possible with degeneration of the 
central nervous system, and in all those cases where a group owes its 
existence to degeneration, the central nervous system takes part in 
the degeneration. 
This law of the paramount importance of the growth of the central 
nervous system for all upward progress in the evolution of animals 
receives confirmation from the study of the development of individuals, 
especially in those cases where a large portion of the life of the 
animal is spent in a larval condition, and then, by a process of trans- 
formation, the larva changes into the adult form. Such cases are 
well known among Arthropoda, the familiar instance being the change 
from the larval caterpillar to the adult imago. Among Vertebrata, 
the change from the tadpole to the frog, from the larval form of 
the lamprey (Ammocetes) to the adult form (Petromyzon), are well- 
known instances. In all such cases the larva shows signs of having 
attained a certain stage in evolution, and then a remarkable trans- 
formation takes place, with the result that an adult animal emerges, 
whose organization reaches a higher stage of evolution than that of 
the larva. 
This transformation process is characterized by a very great 
destruction of the larval tissues and a subsequent formation of new 
adult tissues. Most extensive is the destruction in the caterpillar 
and in the larval lamprey. But one organ never shares in this process 
of histolysis, and that is the central nervous system; amidst the 
ruins of the larva it remains, leading and directing the process of 
re-formation. In the Arthropoda, the larval alimentary canal may 
be entirely destroyed and eaten up by phagocytes, but the central 
nervous system not only remains intact but increases in size, and by 
the concentration and cephalization of its infra-cesophageal ganglia 
forms in the adult a central nervous system of a higher type than 
that of the larva. 
So, too, in the transformation of the lamprey, there is not the 
slightest trace of any destruction.in the central nervous system, but 
simply a development and increase in nervous material, which 
