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As both the Tunicates and the Vertebrates develop from the same chordula, we may infer that there was a corresponding common ancestor of both stems. We may call this the Chordaea, and the corresponding stem-group the Prochordonia or Prochordata. We shall see presently how this conclusion is justified in the present condition of morphological science.
We have first to answer the difficult and much-discussed question of the development of the Chordaea from the Gastraea; in other words, "How and by what transformations were the characteristic animals, resembling the embryonic chordula, which we regard as the common stem-forms of all the Chordonia, both Tunicates and Vertebrates, evolved from the simplest two-layered Metazoa?"
It will be useful first to point out the chief advances in organisation by which the simple Gastraea gradually became the more developed Chordaea. Its bilateral and tri-axial type indicates that the Gastraeads the common ancestors of all the Metazoa divided at an early stage into two divergent groups. Thus arose the typical bilateral form, which has three axes.
In view of its sound establishment and its profound significance, it may very well claim to be a THEORY, and so should be described as the chordonia or chordaea theory. I first advanced this theory in a series of university lectures in 1867, from which the History of Creation was composed.
We must grant, however, that in the whole stem-history of the Vertebrates the long stretch from the Gastraeads and Platodes up to the oldest Chordonia remains by far the most obscure section. We might frame another hypothesis to raise the difficulty namely, that there was a long series of very different and totally extinct forms between the Gastraea and the Chordaea.
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