Darwin accounted for marvelous variety in living forms without invoking arbitrary supernatural forces. It is an approach to biology that leads to experimental questions. Experiment, in turn, leads to discovery, and Darwinism prevails throughout biology because it has been a springboard for so many discoveries. Nevertheless, where determinism of blood and bone soon swept the field, determinism of thought and feeling remains embattled to this day. Wallace's alternative to Darwin remains well entrenched:
The Darwinian theory, even when carried out to its extreme logical conclusion, not only does not oppose, but lends a decided support to, a belief in the spiritual nature of man. It shows us how man's body may have been developed from that of a lower form under the law of natural selection; but it also teaches us that we possess intellectual and moral faculties which could not have been so developed, but must have had another origin; and for this origin we can only find an adequate cause in the unseen universe of Spirit. (Wallace Reference Wallace1889, p. 478)
Darwin's continuity has inspired discoveries based on fresh lines of investigation in field and in laboratory (Boesch Reference Boesch1991; Goodall Reference Goodall1986; Harlow Reference Harlow1949; Koehler Reference Koehler and Winter1925/1959; Russon & Galdikas Reference Russon and Galdikas1993; Watanabe Reference Watanabe, Gardner, Gardner, Chiarelli and Plooij1994, to name only a few). Wallace's noncontinuity has inspired nondiscovery after nondiscovery based on nonexperiments designed to reveal nonhuman failure without revealing anything fresh about the nature of intelligence. Many nondiscoveries of noncontinuity depend on experimental methods that pioneers of comparative psychology discredited roughly one hundred years ago.
Pygmalion in the laboratory
Karl Marbe, for example, studied Basso, a chimpanzee in the Frankfurt zoo, that solved arithmetic problems (Marbe Reference Marbe1917). Asked in German “How much is six plus four?” Basso selected a card bearing the number “10” as in Figure 1. Basso usually chose the correct card. Marbe observed Richard Burkardt, zoo keeper and trainer who drilled Basso on simple problems as one might drill a small school child. Burkardt doubted that Basso knew arithmetic. Asked what he believed Basso was doing, Burkardt said, “She reads my mind.” Marbe tested this hypothesis by asking the trainer to give Basso a series of arithmetic problems and think wrong answers. The result was decisive. Basso repeatedly gave the answer that Burkardt was thinking, regardless of the correct answer in arithmetic. She succeeded at mind reading where she failed at arithmetic.
Array for testing chimpanzee Basso (Marbe 1917). Note that virtually all child versus chimpanzee comparisons cited by Penn et al. allow tester to cue testee in this way.
Had Marbe stopped here, some current theorists might cite Basso as a pioneer demonstration of a chimpanzee with a theory of mind. Instead, Marbe administered further tests showing that Basso selected the card that Burkardt looked at. The trainer himself was unaware that he was gazing at particular cards and was sure that he never gave any hints whatsoever. Earlier, Pfungst (Reference Pfungst and Henry Holt1911/1965) showed that Hans, a German horse, solved arithmetic problems and spelled out German words by following the gaze of human interlocutors who were also unaware that they were hinting. The horse also failed tests when his trainer thought he should fail. That is, the experimenter hints innocently shaped results to conform to experimenter expectations, as a sculptor shapes a lump of clay. After these pioneering studies, experimental procedures to control for inadvertent hints became standard in comparative psychology (B. Gardner & Gardner Reference Gardner, Gardner, Gardner, Gardner and Van Cantfort1989, Fig. 4.1; Harlow Reference Harlow1949, Fig. 1; Warden & Warner Reference Warden and Warner1928).
Oddly, a wave of recent claims of evidence for noncontinuity fail to use any controls for experimenter hints. This failure of method is apparent in virtually all of the experimental evidence that Penn et al. cite. Herrmann et al. (Reference Herrmann, Call, Hernandez-Lloreda, Hare and Tomasello2007) is a very recent example. Fortunately, an online video published by Science clearly shows that experimenters were in full view of the children and chimpanzees they tested. Differences in experimenter expectations or rapport between experimenter and subject easily account for all results.
Interested readers can verify the persistence of this experimental error in evidence of noncontinuity cited throughout Penn et al.'s target article.
Intelligent nature and nurtured intelligence
Additionally, nonexperiments compare caged chimpanzees – lucky if they have a rubber tire to play with or a rope to swing from – with human children from suburban homes. Most modern psychologists would expect caged human children to lose rather than develop cognitive ability. Indeed, the longer chimpanzees live in cages, the lower they score on cognitive tasks (Povinelli et al. Reference Povinelli, Rulf, Landau and Bierschwale1993; Tomasello et al. Reference Tomasello, Davis-Dasilva, Camak and Bard1987).
Credible comparisons depend on comparable conditions. In sign language studies of cross-fostered chimpanzees (R. Gardner & Gardner Reference Gardner, Gardner, Gardner, Gardner and Van Cantfort1989), homelike conditions simulated the rearing environment of human children. Chimpanzees acquired signs in spontaneous conversational interactions with their human foster families the way human children acquire their native languages. Conversations were embedded in the casual interactions of daily life (e.g., Bodamer & Gardner Reference Bodamer and Gardner2002; Chalcraft & Gardner, Reference Chalcraft and Gardner2005; B. Gardner & Gardner Reference Gardner and Gardner1998; R. Gardner & Gardner Reference Gardner, Gardner, Gardner, Gardner and Van Cantfort1989; Jensvold & Gardner Reference Jensvold and Gardner2000; Shaw Reference Shaw2001; Van Cantfort et al. Reference Van Cantfort, Gardner, Gardner, Gardner, Gardner and Van Cantfort1989). They are comparable to dialogues in similar research with human children because cross-fostered chimpanzees and human children carry on conversations under similar conditions.
Patterns of development were comparable to human patterns. Vocabulary, sentence constituents, utterances, phrases, and inflection, all grew robustly throughout five years of cross-fostering. Growth was patterned growth, and patterns were consistent across chimpanzees. Comparable measurements paralleled in detail characteristic patterns reported for human infants (Bodamer & Gardner Reference Bodamer and Gardner2002; B. Gardner & Gardner Reference Gardner and Gardner1998; Jensvold & Gardner Reference Jensvold and Gardner2000; Van Cantfort et al. Reference Van Cantfort, Gardner, Gardner, Gardner, Gardner and Van Cantfort1989). Development was slower than human development without reaching an asymptote.
Mind-numbing drill
Penn et al. discuss the tube-trap problem invented by Visalberghi and Limongelli (Reference Visalberghi and Limongelli1994) as a demonstration of cognitive discontinuity. In fact, Visalberghi and Limongelli subjected their monkeys to highly repetitive drill, a traditional procedure for inducing stereotyped habits that interfere with human as well as nonhuman problem solving. R. Gardner and Gardner (Reference Gardner and Gardner1998, pp. 270–87) discuss decades of evidence for the universally mind-numbing effect of drill on human and nonhuman problem solving found in experiment after experiment for at least a century. Luchins and Luchins (Reference Luchins and Luchins1994) reviews nearly fifty years of closely parallel negative effects of repetitive drill on human problem solving.
