On the Variation of Species, with Especial Reference to the Insecta / Followed by an Inquiry into the Nature of Genera

The author assembles field observations and comparative arguments about variability in insects, documenting that populations often exhibit morphological change and examining probable causes such as climatic and altitudinal influences, episodic heat or cold, soil and habitat differences, and isolation including exposure to storms. The work identifies which organs and characters are most liable to alteration, offers geological reflections on distribution and persistence, and develops a theory of genera intended to help distinguish transient varieties from more permanent taxa while urging closer attention to annulose groups in biogeographic study.