In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as the largest group of organisms in which two individuals are capable of reproducing fertile offspring.
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- Love in animals, has not for its only object animals of the same species, but extends itself farther, and comprehends almost every sensible and thinking being. A dog naturally loves a man above his own species, and very commonly meets with a return of affection.
- The boundaries of the species, whereby men sort them, are made by men.
- John Locke, An Essay Concerning Human Understanding (1689), book III, ch. 6, sec. 37.
- It would be quite false to say that competition is the only relation that obtains between species. Mutual dependence is in general quite as important. Each kind exists within an ecosystem, and needs the others to keep the system going. Thus, grazing animals on the African plains co-exist because each specializes in eating some particular kind of plant, and needs the others to keep the whole pasture at a balanced level. They depend, too, on each other's specialized capacities to give warning of danger. … each also depends for survival on innumerable others, such as the insects which pollinate the plants, the fauna of their intestines and of course their predators. It is unthinkable that any species should be an island.