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Summary.

  • 1
     Two sets of individuals diagnosed on the characters of shell and genitalia as Cepæa hortensis and Cepæa nemoralis have been examined. Each set was obtained from colonies in Wiltshire, Devonshire, Cornwall, Suffolk, Buckinghamshire, and Middlesex.
  • 2
     The following structures were examined: pulmonary vessels, genitalia, kidney, heart, pericardium, and nervous system.
  • 3
     The measurements obtained are presented as they relate to:
  • (a)
     the differences between the colonies of each species;
  • (b)
     the differences between the species as currently designated.
  • 4
     The only absolute distinctions in internal anatomy between the species C. hortensis and C. nemoralis as recognized by the shell-characters are:
  • (a)
     the shape of the dart;
  • (b)
     the arrangement of the mucous glands.
  • 5
     The colonies of each species may differ significantly from one another, when the individual characters are treated statistically.
  • 6
     It is probable that a sample drawn from a few colonies only, even though it contain a large number of individuals, is of little value for the determination of the characters of a larger population.

The differences found, however, are valid for the section studied, and it may be that they represent definite differential tendencies in the population as a whole, which might be correlated with the shell-characters.

  • 7
     Certain organs show a wider range of variability than others: e.g., the pulmonary vessels, genitalia, and kidney are markedly more variable than the heart, pericardium, and nervous system.
  • 8
     The order of variability of the organs is almost identical in the two species and in their colonies.
  • 9
     The various organs do not show the same amount of divergence when those of the one population sample are compared with those of the other. Thus there is more marked divergence between the length of the dart-sac of “nemoralis” individuals and that of “hortensis” individuals than there is between the kidney of the two groups. The following list gives the order (from greatest divergence to least) in which such divergence is manifested in the two samples;—

Length of duct of receptaculum seminis.

Length of dart-sac.

Length of diverticulum.

Length of main pulmonary vein.

Length of kidney.

Pericardium.

Distance between optic nerves.

  • 10
     The organs in C. nemoralis are, on the whole, more variable than those in C. hortensis.