A comparison is outlined between Lineo's and Darwin's theories. The meaning of the word "species" is just slightly different in the two domains. A warning is given to biologists, who daily refer to "species", and who meet contradictions arising from considering species ad both physical (Linneo) and historical (Darwin) terms of reference. Without being aware of the mental processes that are behind the two solutions. An analogy with physics is proposed: Eulero and Lagrange construct their terms of reference, respectively, maintaining elements of space and of time, in order to solve the same problem of mechanics of liquids and gases.