“BALANOGLOSSUS” (AFTER AGASSIZ), AND LARGE SEA LAMPREY (AFTER CUVIER AND HAECKEL), SHOWING GILL-SLITS.—FROM “DARWIN AND AFTER DARWIN” BY ROMANES.
A successful attempt at coming ashore may be seen in the common worm. The worm is still so unacclimatized to land life that instead of living on the earth like other creatures, it lives in it, as if it were a thicker water, and always where there is enough moisture to keep up the traditions of its past. Probably it took to the shore originally by exchanging, first the water for the ooze at the bottom, then by wriggling among muddy flats when the tide was out, and finally, as the struggle for life grew keen, it pushed further and further inland, continuing its migration so long as dampness was to be found. Its cousin the snail, again, goes even further, for it not only carries its shell ashore but when it cannot get moisture, actually manufactures it.
EMBRYOS SHOWING GILL-SLITS.—FROM HAECKEL’s “EVOLUTION OF MAN.”
A. FISH. B. CHICK. C. CALF. D. MAN.
When Man left the water, however,—or what was to develop into Man—he took very much more ashore with him than a shell. Instead of crawling ashore at the worm stage, he remained in the water until he evolved into something like a fish; so that when, after an amphibian interlude, he finally left it, many “ancient and fish-like” characters remained in his body to tell the tale. Now, it is among these piscine characteristics that we find the clue to where Man got his ears. The chief characteristic of a fish is its apparatus for breathing the air dissolved in the water. This consists of gills supported on strong arches, the branchial arches, which in the Elasmobranch fishes are from five to seven in number and uncovered with any operculum, or lid. Communicating with these arches, in order to allow the water which has been taken in at the mouth to pass out at the gills, an equal number of slits or openings are provided in the neck. Without these holes in their neck all fishes would instantly perish, and we may be sure Nature took exceptional care in perfecting this particular piece of the mechanism. Now it is one of the most extraordinary facts in natural history that these slits in the fish’s neck are still represented in the neck of Man. Almost the most prominent feature, indeed, after the head, in every mammalian embryo, are the four clefts or furrows of the old gill-slits.[1] They are still known in embryology by no other name—gill-slits—and so persistent are these characters that children have been known to be born with them not only externally visible—which is a common occurrence—but open, through and through, so that fluids taken in at the mouth could pass through them and trickle out at the neck. This fact was so astounding as to be for a long time denied. It was thought that when this happened, the orifice must have been accidentally made by the probe of the surgeon. But Dr. Sutton has recently met with actual cases where this has occurred. “I have seen milk,” he says, “issue from such fistulæ in individuals who have never been submitted to sounding.”[2]
In the common case of children born with these vestiges, the old gill-slits are represented by small openings in the skin on the sides of the neck and capable of admitting a thin probe. Sometimes the place where they have been in childhood is marked throughout life by small round patches of white skin. These relics of the sea, these apparitions of the Fish, these sudden resurrections, are betrayals of man’s pedigree. Men wonder at mummy-wheat germinating after a thousand years of dormancy. But here are ancient features bursting into life after unknown ages, and challenging modern science for a verdict on their affinities.
When the fish came ashore, its water-breathing apparatus was no longer of any use to it. At first it had to keep it on, for it took a long time to perfect the air-breathing apparatus which was to replace it. But when this was ready the problem was, what to do with the earlier organ? Nature is exceedingly economical, and could not throw all this mechanism away. In fact Nature almost never parts with any structure she has once made. What she does is to change it into something else. Conversely, Nature seldom makes anything new; her method of creation is to adapt something old. Now when Nature started out to manufacture ears, she made them out of the old breathing apparatus. She saw that if 54 water could pass through a hole in the neck, sound could pass likewise, and she set to work upon the highest up of the five gill-slits and slowly elaborated it into a hearing organ.