This paper is dedicated to the memory of our friend and colleague, Chic Richards, whose work significantly advanced our understanding of snail-trematode interactions.

It's a variable life. The remarkable diversity of biological systems is the hallmark of life on earth. Variation served as the basic foundation of Darwin's (1859) view of nature. In the Origin of Species he states, “Among organic beings in a state of nature there is some individual variability; indeed I am not aware that this has ever been disputed.” This incredible diversity attracted many of us to biology. Yet the inherent genotypic and phenotypic variability of biological systems limits our ability to predict the behavior of living systems which, in turn, constrains our search for ‘unifying processes.’

Host-parasite systems are both complex and, not surprisingly, variable. However, at times this fact is not overtly evident. In Darwin's discussion of the variety...

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