This article appeared in The Textbook Letter for March-April 1997,
accompanying reviews of Holt, Rinehart and Winston's high-school
book Biology: Principles and Explorations.
The body of an amphibian is far more complex than that of a fish, and, as you might expect, it takes an amphibian far longer to develop. However, amphibian eggs have about the same amount of yolk as fish eggs, so the amphibian embryo isn't protected in the egg any longer than a fish embryo. Instead, amphibian development takes place in two phases. First, the egg hatches into a larval stage, like some of the larvae found among insects. . . . Only when an individual has grown to a sufficient size does it undergo the second phase of its development -- metamorphosis, a radical developmental transformation into the terrestrial adult form.
The opening claim is an absurdity. We have no way to measure organic complexity, so any claim that a given animal's body is "far more complex" than some other animal's body is bunkum.
So is the rest of the passage, in which Holt's writers fabricate a wholly imaginary difference between amphibians and fishes. The writers want students to believe that a fish acquires its definitive (or "adult") form as soon as it hatches from the egg, without passing through any larval stage; that an amphibian is so "complex" that it must spend some time as a larva before it can attain its definitive form; and that an amphibian, therefore, takes "far longer to develop." None of that has any basis in fact, as we see if we consider some real biology.
To begin, let's notice that some species of fish are viviparous. The embryos do not grow inside eggs, and they do not hatch. The Holt writers' simplistic typology fails to take account of such species in any way. But even if we neglect viviparity and look only at the fishes and amphibians that lay eggs, we find that they do not conform to the writers' formula.
So far, we have accepted the Holt writers' implicit claim that only eggs or larvae "develop" and that the "development" of a fish or an amphibian ends when the animal attains its adult morphology. That claim is false, though, because every animal develops throughout its entire life. One of the signal events in this process is the advent of sexual maturity, and biologists routinely use sexual maturity as an index for comparing rates of development. If we do this, we again find no support for the writers' statement that "it takes an amphibian far longer to develop."
In fishes and amphibians alike, some species reach sexual maturity in a matter of months -- but among fishes and amphibians alike, maturation usually takes a year or longer. It seems to be especially slow in certain fishes: Some mackerel sharks must grow for twelve years before they can reproduce, and maturation in the big, cold-water sturgeons apparently takes a decade or two. I am not aware of any comparable cases among amphibians.
The lesson here is the usual one: Nature is much more interesting than the simple-minded twaddle beloved by schoolbook-writers. Evolution has produced a living world that teems with diversity, disparity, convergence and coincidence, beggaring the pat, anthropocentric scheme of the ladder.
William J. Bennetta is a professional editor, a fellow of the California Academy of Sciences, the president of The Textbook League, and the editor of The Textbook Letter. He writes frequently about the propagation of quackery, false "science" and false "history" in schoolbooks.
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