Definition of "subspecies"

I believe it [the trilobite Calymene blumenbachii] has been divided into several forms, which cannot claim more than the rank of subspecies, as they pass by slight gradations into forms which no author has yet thought it right to separate from the type species.

1866, A[ndrew] C[rombie] Ramsay, “Trilobita”, in The Geology of North Wales (Memoirs of the Geological Survey of Great Britain and of the Museum of Practical Geology; [III]), London: Printed for Her Majesty's Stationery Office; published by Longmans, Green, Reader, and Dyer, page 325

But these slight differences will not justify us in considering the West African chiquera as a true species distinct from the Indian true chiquera; it is only a subspecies of the latter true species.

1851, [Johann Jakob] Kaup, “Descriptions of Some New Birds in the Museum of the Earl of Derby”, in Proceedings of the Zoological Society of London, part XIX, London: Printed for the Society; sold at their house in Hanover Square, and by Messrs. Longman, Brown, Green, and Longmans, Paternoster Row, page 42

Granular Augite. […] This subspecies of augite has been hitherto found principally in primitive rocks at Arendal in Norway, in several of the iron-mines, particularly that named Ulve-Grube.

1821, Robert Jameson, Manual of Mineralogy: Containing an Account of Simple Minerals, and also a Description and Arrangement of Mountain Rocks, Edinburgh: Printed for Archibald Constable & Co.; London: Hurst, Robinson & Co., page 166