As Quote Investigator explains, allegories about animals doing impossible things have been incredibly popular in the past century. But no, this one isn't from Einstein. (Source: http://quoteinvestigator.com/2013/04/06/fish-climb/.)
Misattributed
Variant: Everybody is a genius. But if you judge a fish by its ability to climb a tree, it will live its whole life believing that it is stupid.
“Einstein was just a genius of theoretical physics. If you introduced him to biology or chemistry, he would look like a fish trying to climb a tree.”
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Mwanandeke Kindembo 1044
Congolese author 1996Related quotes
Seattle: University of Washington Press, 1966, p. 10.
Of Molecules and Men (1966)
Interview on Rileys' News http://www.rileys.co.uk/news/240.
Source: Translations, Monkey: Folk Novel of China (1942), Ch. 2 (p. 22)
Source: Education as a Science, 1898, p. 298.
"What is Science?" http://orwell.ru/library/articles/science/english/e_scien, Tribune (26 October 1945)
1900s, God Does Not Exist (1904)
Introduction Note: Max Planck, "Acht Vorlesungen iiber theoretische Physik" (1910)
Higher Mathematics for Chemical Students (1911)
“That life is chemistry is true but boring, like saying that football is physics.”
Source: Genome (1999), Chapter 1 “Life” (p. 15)