Variation in animals and plants . a mathematical expression can be found. There is stilla third group of cases, however, in which the curve ofdistribution is, as a rule, very asymmetrical, but forwhich, even if symmetrical, no single general mathe-matical expression is obtainable. A study of suchcurves has taught us that the cause is frequently refer-able to the fact that our material is not homogeneous;that, in fact, we have a mixture of varying numbers oftwo or more groups of individuals differing in meansize and range of variation. For instance, Bateson*measured the length of the forceps of
Variation in animals and plants . a mathematical expression can be found. There is stilla third group of cases, however, in which the curve ofdistribution is, as a rule, very asymmetrical, but forwhich, even if symmetrical, no single general mathe-matical expression is obtainable. A study of suchcurves has taught us that the cause is frequently refer-able to the fact that our material is not homogeneous;that, in fact, we have a mixture of varying numbers oftwo or more groups of individuals differing in meansize and range of variation. For instance, Bateson*measured the length of the forceps of 583 specimens ofthe common earwig, Forficula auricularia, which had * Materials for the Study of Variation, p. 41. 37 38 DISCONTINUOUS VARIATION. been collected at random in one day in the FameIslands. Only mature males with elytra fully devel-oped were measured. The range of variation was to mm., the various lengths occurring with afrequency indicated by the accompanying curve. Here 120 100 3I 80 a M *M o 60 40 SO 0.
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