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Index to Creationist Claims,  edited by Mark Isaak,    Copyright © 2006
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Claim CB805:

Since evolution says organisms came from a common ancestor and since they lived in a continuity of environments, we should see a continuum of organisms. There should be a continuous series of animals between cats and dogs, so that one could not tell where cats left off and dogs began.


Morris, Henry M. 1985. Scientific Creationism. Green Forest, AR: Master Books, pp. 70-71.


  1. The claim might be true if there were no such thing as extinction. But since species do become extinct, intermediates that once existed do not exist today. Since extinction is a one-way street, species can only become less connected over time. This is clear if we look at the fossil record, in which early members of separate groups are much harder to tell apart.

  2. Environments (and ecological niches) are not really as continuous as the claim pretends. Dogs bring down their prey through long chases, and cats ambush their prey; dogs are made for long-distance running, and cats are made for short sprints with high acceleration from a standing start. These requirements are quite different, and it is hard to achieve both in a single body. Compromises between the two have disadvantages in competition with specialists for either type, and thus natural selection culls them. Intermediates are competitive only so long as specialists are absent; so when specialists evolve, the intermediates are likely to become extinct.

  3. In part, distinctness is an illusion caused by our choice of which groups to give names to. Groups with unclear boundaries tend not to get separate names, or groups in which intermediate forms exist are chopped in half arbitrarily (especially obvious if fossil forms are considered; e.g., the line between dinosaurs and birds is arbitrary, increasingly so as new fossils are discovered).

  4. There are indeed several cases of continua in nature. In many groups, such as some grasses and leafhoppers, different species are very hard to tell apart. At least ten percent of bird species are similar enough to another species to produce fertile hybrids (Weiner 1994, 198-199). The most obvious continua are called ring species, because in the classic case (the herring gull complex) they form a ring around the North Pole. If we start in Western Europe and move west, similar populations, capable of interbreeding, succeed each other geographically. When we have traveled all the way around the world and reach Western Europe again, the final population is different enough that we call it a separate species, and it is incapable of interbreeding with herring gulls, even though they are connected by a continuous chain of interbreeding populations. This is a big problem for creationists. We expect kinds to be easily determined if they were created separately, but there are no such obvious divisions:
    They are mistaken, who repeat that the greater part of our species are clearly limited, and that the doubtful species are in a feeble minority. This seemed to be true, so long as a genus was imperfectly known, and its species were founded upon a few specimens, that is to say, were provisional. Just as we come to know them better, intermediate forms flow in, and doubts as to specific limits augment. (de Condolle, quoted in Darwin, 1872, chap. 2)


  1. Darwin, C., 1872. The Origin of Species, 1st Edition. Senate, London.
  2. Weiner, Jonathan, 1994. The Beak of the Finch: a story of evolution in our time. New York: Knopf.

Further Reading:

Hazard, Evan B. 1998. Teaching about "intermediate forms." The American Biology Teacher 60(5): 359-361.

Darwin, C., 1859. The Origin of Species, 1st Edition. Senate, London.
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created 2003-4-26, modified 2006-4-23