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Updated: May 23, 2025
Both Darwin's vertebral plexus, and Herbert Spencer's "line of individuation," must begin with the lancelet and its disputed head, and end in the Catarrhine or Old World monkey. No a priori induction will ever extend this line or plexus to man.
Lastly, one single member of the immense and diversified class of fishes, namely, the lancelet or amphioxus, is so different from all other fishes, that Haeckel maintains that it ought to form a distinct class in the vertebrate kingdom.
Recently eight to ten species of the amphioxus have been determined, distributed in two or three genera. Transverse section of the head of the Amphioxus. Johannes Muller classed the lancelet with the fishes, although he pointed out that the differences between this simple vertebrate and the lowest fishes are much greater than between the fishes and the amphibia.
We must also admit that there is a much wider interval in mental power between one of the lowest fishes, as a lamprey or lancelet, and one of the higher apes, than between an ape and man; yet this interval is filled up by numberless gradations.
The water is very deep and of a fine olive green, and, being remarkably clear, the light stones lying at the bottom are distinctly visible, among which at my last visit we could descry great fishes, probably bass, pursuing shoals of launces." By "launces" the writer meant what we should now call the lancelet. Just south of Dollar is the old smugglers' cave known as Raven's Hugo.
The radical agreement of the lancelet and the sea-squirt in the first and most important stages of development shows something more than their close anatomic affinity and their proximity in classification; it shows also their real blood-relationship and their common origin from one and the same stem-form. In this way, it throws considerable light on the oldest roots of man's genealogical tree.
Man is only a more advanced stage of the vertebral type that we find unmistakably in the Amphioxus in its characteristic features. We need only recall the picture of the ideal Primitive Vertebrate given in a former chapter, and compare it with the lower stages of human embryonic development, to convince ourselves of our close relationship to the lancelet.
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