The Phylogenetic Scale

Implicit in the work of comparative psychologists from Darwin's day until about the 1930s is something called the phylogenetic scale This is a ranking of animals according to their general complexity and ability, from lowest to highest. The concept is still present in our language, when we use the phrase "lower animals."

What was the "phylogenetic scale"? How did it resemble the Scala Naturae of Aristotle?

The idea that some species are more advanced than others goes back nearly 2400 years to Aristotle's notion of a Great Scale of Nature or Scala Naturae. Aristotle suggested that animals could be ordered from least (for example, worms and snails) to intermediate levels (such as dogs and cats) to the highest and most advanced level (humans). Romanes, like Darwin, accepted this idea as common sense. He wanted to trace the development of intelligence as it moved "up the phylogenetic scale" from simple or primitive animals to complex or advanced animals.

On the surface, the phylogenetic or "phyletic" scale seems reasonable. It is true some species are more advanced than others in particular ways. However, modern psychologists recognize many different forms of intelligence. There could be a different phylogenetic scale for each type of intelligence. On a test of odor recognition, a bloodhound would rate as far more advanced than a human.

Why does the Scala Naturae make no sense from an evolutionary perspective?

The phylogenetic scale makes no sense from an evolutionary perspective. Putting the animals in a linear order (rat, cat, monkey, human) suggests that a rat, if it got a little smarter, would think like a cat. A smart cat might think like a monkey, and a smart monkey might think like a dull human. But why should this be so? The animals are not ancestral to each other and do not represent a single evolving lineage. No cat ever descended from a rat, no monkey was ever descended from a cat, and no human ever descended from a chimpanzee. All these species have been evolving independently for millions of years. Different currently-existing species do not grow into each other, and there is no reason to expect their mental capacities to fall into a smooth progression (Hodos & Campbell, 1969).

However, Romanes believed in the principle of continuity, which said all animals think the same way and differed only in speed of learning. If that was the case, then it made sense to rank animals according to how fast they learned, from least to best, earthworm to human. Today's animal researchers no longer endorse the principle of continuity. Instead, they are respectful of each species' unique forms of intelligence.

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Copyright © 2007 Russ Dewey