The Origin of Vertebrates

by Walter Holbrook Gaskell

What applies to the vertebrate phylum applies also to the invertebrate groups. Here also an upward progress is recognized as we pass from the sponges to the arthropods—a progress which is manifested, 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 transformation, 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 (Ammocœtes) 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 transformation 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-œsophageal 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 results in the formation of a brain region more like that of the higher vertebrates than exists in Ammocœtes.

In these cases the development is upward—the adult form is of a higher type than that of the larva. It is, however, possible for the reverse to occur, so that the individual development leads to degeneration, not to a higher type. Instances are seen in the Tunicata, and in various parasitic arthropod forms, such as Lernæa, etc. In these cases, the transformation from the larval to the adult form leads to degradation, and in this degradation the central nervous system is always involved.

It is perhaps a truism to state that upward progress is necessarily accompanied by increased development of the central nervous system; but it is necessary to lay special stress upon the importance of the central nervous system in all problems of evolution, because there is, in my opinion, a tendency at the present time to ignore this factor to too great an extent.

The law of progress is this—The race is not to the swift, nor to the strong, but to the wise.

This law carries with it the necessary corollary that the immediate ancestor of the vertebrate must have had a central nervous system nearly approaching that of the lowest undegenerated vertebrate. Among all the animals living on the earth at the present time, the highest invertebrate group, the Arthropoda, possesses a central nervous system most closely resembling that of the vertebrate.

The law, then, of the paramount importance of a steady development of the central nervous system for the upward progress of the animal kingdom, points directly to the arthropod as the most probable ancestor of the vertebrate.

Evolution of Tissues.