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71 stress. Given long time periods, suggested anthropologist Richard Lee, evolution is a means for generating highly improbable results.'° Mind-Stretcher. This debate has implicitly assumed that the basic princi- ples of evolution we know—genetic variation and natural selection—will operate on other worlds. Genes and chromosomes may not be the only Evolutionist Simon Conway Morris, a frequent rival of Gould, saw evolu- tion as seeded with probabilities, if not inevitabilities. “Convergence is ubiquitous from molecules to social systems,” he declared. “In fact, the study of convergence reveals a deep structure to life. This strongly suggests that what is true on Earth is true anywhere.” Morris challenged the belief that the immense number of possibilities confers an inherent unpredictability on evolution. The main principle of evolution, beyond selection and adaptation, is the drawing of new plans but relying on the tried and trusted building blocks of organic architecture. Life is full of inherencies.'® As Morris saw it, convergent evolution, broadly speaking, produces pre- dictable evolutionary outcomes. Multicellularity has evolved repeatedly, warm-bloodedness several times. Mechanisms used by diverse organisms to see, smell, hear, echolocate, sense electrical fields, and maintain balance are often convergent. For all of life’s plenitude there is a strong stamp of limitation, imparting not only a predictability to what we see on Earth, but by implication elsewhere. The evolutionary routes are many, concluded Morris, but the destinations are limited.” Gould recognized that “architectural constraints” limit adaptive scope and channel evolutionary patterns. While emphasizing that evolution is contingent, Gould also stressed the importance of emergence—the idea that novelties are present in complex phenomena that can neither be found in, nor explained by, simpler processes.'® Molecular biologist Sean Carroll found a long history of support for the general notion of overall evolutionary trends toward increases in size, complexity, and diversity. Scientists have proposed two fundamentally dis- tinct mechanisms to explain these trends. One is a random, passive ten- dency to evolve through an overall increase in variance. The other is a nonrandom, active or driven process that biases evolution toward increased size and complexity. Multicellularity evolved independently many times, Carroll emphasized. Once it did, macroscopic forms arose with new body plans or physiologies and with greater degrees of morphological complexity. The emergence of new forms was often followed by periods of rapid diversification. Although global trends may be passive, there may be active, directional trends nested within the overall arc of evolutionary history. Assuming a cellular basis of Chancists Versus Convergionists way that biological information can be organized.