The difficulty, then, of forming a new digestive system in the passage from the arthropod to the vertebrate is very much the same as the difficulty in forming a new respiratory system in the passage from the water-breathing fish to the air-breathing amphibian--a change which does not strike us as inconceivable, because we know it has taken place.
The whole argument so far leads to the conclusion that vertebrates arose from ancient forms of arthropods by the formation of a new alimentary canal, and the enclosure of the old canal by the growing central nervous system. If this conclusion is true, then it follows that we possess a well-defined starting-point from which to compare the separate organs of the arthropod with those of the vertebrate, and if, in consequence of such working hypothesis, each organ of the arthropod is found in the vertebrate in a corresponding position and of similar structure, then the truth of the starting-point is proved as fully as can possibly be expected by deductive methods. It is, in fact, this method of comparative anatomy which has proved the descent of man from the ape, the frog from the fish, etc.
Let us, then, compare all the organs of such a low vertebrate as Ammocoetes with those of an arthropod of the ancient type.
{ 59}LIFE HISTORY OF THE LAMPREY--NOT A DEGENERATE ANIMAL.
The striking peculiarity of the lamprey is its life-history. It lives in fresh water, spending a large portion of its life in the mud during the period of its larval existence: then comes a somewhat sudden transformation-stage, characterized, as in the lepidopterous larva, by a process of histolysis, by which many of the larval tissues are destroyed and new ones formed, with the result that the larval lamprey, or Ammocoetes, is transformed into the adult lamprey, or Petromyzon. This transformation takes place in August, at all events in the neighbourhood of Cambridge, and later in the year the transformed lamprey migrates to the sea, grows in size and maturity, and returns to the river the following spring up to its spawning beds, where it spawns and forthwith dies. How long it lives in the Ammocoetes stage is unknown; I myself have kept some without transformation for four years, and probably they live in the rivers longer than that before they change from their larval state. It is absolutely certain that very much the longest part of the animal's life is spent in the larval stage, and that with the maturity of the sexual organs and the production of the fertilized ova the life of the individual ends.