Origin of Species Variorum

their sense-organs, in their heart and system of circulation, in the position of every tuft of hair with which their stomachs, equally complicated in both cases, are lined, and lastly in the water-breathing branchiæ, even to the microscopical hooks by which they are cleansed. Hence it might have been expected from mere analogy that the equally important air-breathing apparatus would have been the same in the few species in both families which are thus furnished; and this might have been the more confidently expected by those who believe in the creation of each separate species; for why should this one apparatus, given for the same special purpose to a few species which are so closely similar or rather identical in all other important points, have been made to differ?

Fritz Müller then argued to himself that this close similarity in so many points of structure must, in accordance with the views advanced by me, be accounted for by inheritance from a common progenitor. But as the vast majority of the species in the above two families, as well as the main body of crustaceans of all orders, are aquatic in their habits, it is improbable in the highest degree, that their common progenitor should have been adapted for breathing air. Müller was thus led carefully to examine and describe the apparatus in the few air-breathing species; and in each he found it to differ in several important points, as in the position of the orifices, in the manner in which they are opened and closed, and in some accessory details. ↑ Now, on the belief that species belonging to distinct families, already differing in some characters, and which whenever they varied would probably have varied in different manners, have been slowly adapted through natural selection to live more and more out of water and to breathe the air, it is quite intelligible, and might even have been con-