Most of the theories were mutually exclusive; none of them were based on detailed comparisons of several systems of organs; and none of them threw any light on vertebrate anatomy, or afforded even an approximate solution of the real problem.
The vital spark in these theories vanished long ago, but certain basic postulates in them have slowly petrified into the semblance of established facts and have introduced into morphology many false ideas, and a system of nomenclature that implies a knowledge we by no means possess. Many a zoologist, proud of his adherence to sound zoological principles, accepts these familiar terms as evidence that the things so named are really what the names imply, as, for example, the terms dorsal and ventral, right and left, gastrula, archenteron, blastopore and cœlomic pouches. The same subtle transformation of theory into fact is shown by the perpetuation of the view that the notochord is made of endoderin because it arises from the walls of the "archenteron"; and by the one that the primitive streak represents the closed lips of an "Uhrmund."
When real progress along the old lines ceased, the problem came to be regarded as hopeless, largely because it was assumed that the ancestors of the vertebrates were small, soft-bodied animals, unlikely to be preserved as fossils. The "practical" biologist then turned his attention to cytology, experimental evolution and genetics, and the study of morphology and phylogeny became almost as discreditable an occupation, especially in the eyes of the new school of biologists in this country, as the study of metaphysics, or the description of new species.
During this long search for the ancestors of the vertebrates, the arthropods (insects, Crustacea and arachnids), the largest and most highly organized class of invertebrates were altogether excluded, with astonishingly aggressive unanimity, from their due consideration. It is difficult to understand this impenetrable state of mind, for it did not appear to be based on any known facts, or upon any positive evidence whatever. It was apparently due to a widespread conviction that the general trend of evolution in the arthropods did not lead toward the vertebrates, that the arthropods themselves were too highly specialized to give rise to a new type, and to the fact that the hue and cry of the annelid theory was leading the chase in another direction.
In view of this situation, it may be readily understood that another attempt to connect the genealogy of the vertebrate stock with that of the invertebrates will now have to contend with a widespread indifference born of repeated failures; with interests already diverted into other channels; and with that first, unreasoning hostility that is the protective attitude of the mind toward any strange idea that threatens to steal away our cherished convictions.
