TITLE: Do mathematical objects disappear?
QUESTION [64 upvotes]: I am asking this question starting from two orders of considerations. 
Firstly, we can witness, considering the historical development of several sciences, that certain physical entities "disappeared": it is the case of luminiferous aether with the surge of Einstein's relativity, or the case of the celestial spheres that disappeared in the passage from the pre-copernica universe to the Copernican universe.
Secondly, if we consider the evolution of mathematics, we assist to quite an opposite phenomenon: new mathematical objects are often invented and enrich already existing ontologies (let us consider the growth of the number system with negative, imaginary, etc., or the surge of non-euclidean geometries).
But can anyone think of examples of mathematical objects that have, on the contrary, disappeared (have been abandoned or dismissed by mathematicians)?

REPLY [8 votes]: There are cases like this ...
Walter Feit and John Thompson wrote a 250-page paper on nonabelian simple groups of odd order, and in the end conclude that there are none.