text: Brood size decreases with (adult) body mass, so that an elephant has fewer young (usually just one) per brood than a mouse.Lifespan increases with gestation period, and also with body mass, so that elephants live longer than mice, have a longer period of gestation, and are heavier.As a final example, fecundity decreases with lifespan, so long-lived kinds like elephants have fewer young in total than short-lived kinds like mice.Classification of living things Aristotle distinguished about 500 species of animals, arranging these in the History of Animals in a graded scale of perfection, a nonreligious version of the scala naturae, with man at the top.His system had eleven grades of animal, from highest potential to lowest, expressed in their form at birth: the highest gave live birth to hot and wet creatures, the lowest laid cold, dry mineral-like eggs.Animals came above plants, and these in turn were above minerals.He grouped what the modern zoologist would call vertebrates as the hotter "animals with blood", and below them the colder invertebrates as "animals without blood".Those with blood were divided into the live-bearing (mammals), and the egg-laying (birds, reptiles, fish).Those without blood were insects, crustacea (non-shelled – cephalopods, and shelled) and the hard-shelled molluscs (bivalves and gastropods).He recognised that animals did not exactly fit into a linear scale, and noted various exceptions, such as that sharks had a placenta like the tetrapods.To a modern biologist, the explanation, not available to Aristotle, is convergent evolution.Philosophers of science have generally concluded that Aristotle was not interested in taxonomy, but zoologists who studied this question in the early 21st century think otherwise.He believed that purposive final causes guided all natural processes; this teleological view justified his observed data as an expression of formal design.