Kostitzin, Vladimir Alexandrovitch. “Symbiosis, parasitism and evolution.” In *The Golden Age of Theoretical Ecology: 1923–1940* , pp. 369-408. Springer, Berlin, Heidelberg, 1978.

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My purpose here is to elucidate the mathematical theory of symbiosis and parasitism with some remarks on the theory of evolution. The equations which I consider are little more than an initial and very gross approximation. The difficulties one encounters in trying to construct a rational or mathematical biology are enormous, and the prejudices almost discouraging. The first and the largest of these difficulties is the almost complete absence of quantitative data. With the exception of a few fortunate cases of practical importance, one finds no data, even imprecise and scattered in time, on animal or plant associations. Consequently, all attempts at theory are severely limited by the impossibility of experimental verification.