1 8 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 (Ammocwtes) 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 



