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 Observational science has a bad reputation among some scientists, for several reasons. First, it cannot change experimental conditions as some other fields can. Second, its initial stage is often ‘just’ data gathering, a ‘fishing expedition’, because the phenomenon is still so incompletely understood that few hypotheses are available to channel observations. Third, and probably most important, the initial stage of most observational sciences is qualitative -- subjective -- not quantitative. Often the system is so complex and so many parameters can be measured quantitatively, that the scientist cannot discern which characteristics should be measured. I suspect that astronomy enjoys a much higher reputation than clinical psychology among scientists because it has progressed farther along a continuum, which begins with overwhelming complexity, progresses to pattern recognition and qualitative hypothesis testing, and culminates in quantitative testing of theoretical models.

Some scientists whose research is amenable to carefully designed experiments think that any research lacking such control is ‘less scientific’ than their own research. Nothing could be farther from the truth, and one should beware the assumption that the same standards for scientific method apply to all types of science. “If one has only a hammer, one tends to look at everything as if it were a nail.” “More discoveries have arisen from intense observation of very limited material than from statistics applied to large groups. The value of the latter lies mainly in testing hypotheses arising from the former.” [Beveridge, 1955] The following late 19th-century experiment by naturalist J. Henri Fabre [Teale, 1949] is worth a detailed description because it illustrates both the power of observational techniques and the remarkable impact that the subtlest experimental intervention can have on the value of a suite of observations. It should be noted that Fabre tried several similar, unsuccessful experiments before he recognized an emerging opportunity and seized it. To what extremes can animals be enslaved by instinct? Fabre investigated this question by studying the pine processionary, a moth caterpillar that leaves a silky thread behind it as it travels. Nearly always, it chooses to follow existing silky paths, laid down by other pine processionaries. Usually this strategy is valuable for survival, leading each individual among the established food supplies of pine needles.

Watching a parade of pine processionaries approach the rim of a palm vase in his greenhouse, Fabre waited until the leader had completed a full circle around the rim. He intercepted the parade, by quickly brushing away all caterpillars and trails below the rim. That was the extent of his experimental intervention. He then observed the result, a continuous, leaderless string of caterpillars rimming the pot.

Because pine processionaries simply follow the silky trails left by others, the caterpillars kept going in a circle. Night and day, they kept going in the same circle. Only during the coldest part of the night did they slump. When they started again on the third day, they were huddled in two groups. Two leaders started the march around the rim, but soon the two groups combined into a continuous ring. On the fourth day the first to wake had slumped off the track. It and six followers entered the new territory of the pot’s interior, but they found no food and eventually wandered back to the rim, retaking the circular path. On the fifth day, a leader and four followers strayed from the path and explored the outside of the vase, to within nine inches of a pile of pine needles. They failed to notice this food and wandered back to the rim. Two days later, now staggering from hunger, one wandered and found the pine needles. Eventually the rest of the group followed. The pine processionaries had circled hundreds of times over a seven-day period and failed to recognize the uselessness of their circular path. They followed instinct to the point of collapse repeatedly, surviving only by chance.