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Excerpt from Selection and Cross-Breeding in Relation to the Inheritance of Coat-Pigments and Coat-Patterns in Rats and Guinea-Pigs
It is generally agreed that the course of evolution is largely in¿uenced by two factors, variation and heredity; but Opinions differ as to what sorts of variation have evolutionary significance and as to the manner of their inheritance.
It has been recognized by several investigators that variations are of two distinct sorts. Bateson has called these two sorts of variation contin nous and discontinuous; more recently De Vries has called them ¿uctuations and mutations, respectively.
By continuous variation (or ¿uctuation) we understand ordinary indi vidual variation within a species. The individuals differ among themselves in size, color, and other structural features. By examining a considerable number of them we can form an idea Of What is the commonest (or modal) condition as regards each structural feature; and likewise what is the average (or mean) condition.
Usually, but not always, the modal and mean conditions are approximately the same, and any other condition is the less frequent in occurrence, the greater its deviation from them. It follows that the most extreme condi tion Observed is connected with the most usual (or modal) condition by an unbroken series of intermediate conditions, and we may call the series as a whole continuous. The distribution of the individuals in such a series is governed by the laws of chance, and may be successfully analyzed by statistical methods.
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