Linné on line 
Mathematics in Linnaeus’ time Mathematics past and present Euclid
Euclid
It is believed that Euclid lived about 300 B.C. in Alexandria – at that time a center of science, with its fantastic library. Euclid's most famous work, Elements, contains virtually all the mathematics Greek scholars had arrived at up to that time. A major share of Elements is about geometry.
Euclidean geometry is geometry on a plane. Here, the sum of angles in a triangle is 180° and a line can be extended to any length whatsoever without meeting itself.
Non-Euclidean geometry is geometry on other surfaces than a plane. For example, a triangle drawn on a sphere has a sum of angles greater than 180°. A line extended far enough will meet itself. On a sphere, a line is thus a circle. Theories for non-Euclidean geometry were developed in the 19th century by Lobachevsky and Bolyai, among others.
Triangle on a sphere (non-Euclidean geometry) and on a plane (Euclidean geometry).
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