The science of comparative and integrative physiology stems from two main intellectual roots - mechanism and evolution. Mechanistic comparative physiology, historically the first and initially certainly the dominant of these two approaches, uses organisms as an experimental parameter per se, taking advantage of lineage-specific characteristics to help figure out how fundamental biological processes work. Today, by absorbing numerous molecular biology tools and conceptual frameworks, this philosophically traditional approach to physiology is given renewal, strength, and excitement. Evolutionary physiology, the second major root from which the field grows, early on aimed to sort out major evolutionary pathways of physiological systems. The tips of phylogenetic trees were examined in enormous detail; then the branches were sketched in. forming hypothetical (some would even say, imaginary) evolutionary pathways. Today modern biologists utilize more rigorous criteria for working out the branching details in the evolution of physiological systems; in cases such as the diving response in pinnipeds, the co-evolution of components of complex physiologies is beginning to be illuminated. Fusing these two main streams of current physiology seems to present perhaps the most vigorous trajectory of our discipline into the next century.
