Although much has been written about the theological conflicts with Darwinian theory, little is known of the powerful scientific objections that modified Darwin’s beliefs.
During Darwin’s lifetime, the accepted theory of heredity was not Mendel’s theory of particulate inheritance , which, though published, was unrecognized, but the theory of blending inheritance , whereby characters are inherited in heterozygous individuals that show the effects of both alleles. As a result the inherited characters in the offspring are intermediate between those of the parents.), which holds that forms intermediate between those of the parents result from mating. Jenkin pointed out that if a rare and favorable mutation occurred, it would soon be blended out by repeated crossings from the wild-type form. Disputing Darwin’s conception of evolution as proceeding through the natural selection of those with slightly better characteristics that arose randomly, Jenkin concluded that natural selection could not account for the tremendous diversity of life, hypothesizing that large numbers of organisms mutated simultaneously in the same direction—a controlled orthogenetic process resembling a series of “special creations.”
Since “special creationism” was an ideological target of his, Darwin found himself in a quandary. Although he did not abandon his theory, he admitted that natural selection played a much smaller part in evolution than he had previously claimed. He also embraced the Lamarckian concept that acquired traits in parents are transmitted to their offspring, thus providing a mechanism by which an entire population could change in the same direction at once.
Another potent objection came from the physicists led by Lord Kelvin, who contested the assumption of previous geologists and biologists that life had existed for billions of years, if not infinitely. How, they asked, could evolution proceed by slow steps in millions of years, and how could advanced forms recently evolved show such great differences? The Kelvinists, basing their conclusion on the assumption that the sun was an incandescent liquid mass rapidly radiating heat, calculated that the age of the earth was between 20 and 40 million years.
Admitting that their calculations were correct and their premises rational, Darwin was forced to adjust this theory. He proposed that change had occurred much more rapidly in the past than in the present, where species seemed static, and that more advanced forms varied more rapidly than lower forms. This provided further reason to advocate Lamarck’s theory of inheritance, because that could account for the rapid change.
Interestingly, both these retreats of Darwin were later shown to be faulty. The discovery that the sun runs on a nearly infinite amount of atomic fuel totally invalidated Kelvin’s argument, Mendel was “rediscovered” in the twentieth century, when it was pointed out that the particulate nature of inheritance meant that favorable mutation not only could persist, but could rapidly become prevalent.
Question: The primary purpose of the passage is to
- outline the process by which Darwin formulated and modified his theory of natural selection
- propose a new interpretation of Darwin’s theory of evolution
- explain how other scientists of the time helped Darwin modify and perfect his theories
- defend Darwinian theory against the objections raised by Darwin’s contemporaries in the scientific community
- discuss some of the scientific controversy that Darwin sparked and describe his response to it
Question: It can be inferred from the passage that the theory of blending inheritance would predict that the offspring of
- two strains of snapdragons, one with abnormal, radically symmetrical flowers and the other with normal, bilaterally symmetrical flowers, would always have normal, bilaterally symmetrical flowers
- a white horse and a black horse would always be gray
- a man with type A blood and a woman with type B blood would always have type A, type B, or type AB blood
- a fly with large eyes and a fly with small eyes would always have one large eye and one small eye
- two pink-flowered plants would always be red or white
Question: It can be inferred from the passage that “wild-type” means
Question: Which of the following, if it could be demonstrated, would tend to support the Lamarckian concept that Darwin embraced?
- Human beings evolved from now-extinct animals much like chimpanzees as a result of an erratic accumulation of changes in the gene pool through thousands of generations.
- Some parental traits disappear in offspring and reappear in the following generation.
- All species of organisms were immutably created in their present forms.
- Rats who have had their trails cut off produce tailless offspring.
- Those hereditary traits that make their owners more likely to grow up and reproduce become increasingly common in a population from one generation to the next.
Question: The author’s attitude toward Jenkin and Kelvin can best be described as
Question: According to the passage, Darwin modified his beliefs in order to
- bring them into line with the theory of particulate inheritance
- disprove Lord Kelvin’s view on the age of the earth
- meet the objections of Jenkin and Lamarck
- resolve theological conflicts about evolution
- dissociate himself from those who believed in “special creationism”
Question: The author sets off the word “rediscovered” in quotation marks in order to
- emphasize that major scientific theories are rarely acknowledged or accepted when they are first promulgated
- indicate that the term is somewhat ironic, since Mendel’s work was virtually ignored when it was published
- rebuke the scientific community for deliberately suppressing Mendel’s work until long after his death
- underscore the similarity between Mendel’s theory of particulate inheritance and the theory of blending inheritance that was accepted during his lifetime
- suggest that a scientist of Darwin’s stature should have read Mendel’s work when it was first published and immediately recognized its importance
Question: It can be inferred from the passage that if Mendel’s work had been recognized and accepted during Darwin’s lifetime, it would have had which of the following effect?
- I. It would have refuted Jenkin’s objections to Darwin’s theories.
- II. It would have supported Darwin’s theory that evolution proceeds by very slow steps over millions of years.
- III. It would have clarified and supported Darwin’s theory of natural selection.
- I only
- III only
Question: All of the following can be reasonably inferred from the passage EXCEPT:
- The idea that evolution occurs by means of natural selection was not widely accepted until the twentieth century.
- Darwin’s theories were originally predicated on the assumption that the earth is more than 40 million years old.
- Many of Darwin’s ideas about heredity were later shown to be incorrect.
- Other scientists of Darwin’s time, including both Jenkin and Lamarck, believed in evolution.
- Darwin was the only scientist of his day who believed in natural selection.
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