New species have, however, been discovered by the explorers of the new Western States and in Tropical America, where, in the sparsely-settled districts, they still come into houses as of yore, and where the rattlesnake campaign is still an annual sport for the venturesome pioneers. In 1872, two thousand of the species _Crotalus confluentus_ were killed in the Yellowstone Region. One other question in the history of the rattlesnake—‘Does it swallow its young in times of danger?’ or more correctly speaking, ‘Does it receive its young into its œsophagus as a place of safety?’—is considered in chap. xxvii. Other discussions of modern times, both in assemblies of zoologists and through printed correspondence, have been on the rattle, when and why vibrated, how affected by damp, etc., all claiming a place in rattlesnake history, but considered elsewhere in this work. A whole volume might be written on this rattling tail, evolved out of the scant materials of the sixteenth century into the prolific matter of the nineteenth. You can scarcely take up one of the many scientific journals of the United States, in which zoology forms a part, without finding mention of a rattlesnake. Within a very few years the subject has been popularized in our own zoological journals also. In connection with the venom come of course the cures, concerning which the experiments of Dr. Weir Mitchell form a notable point in rattlesnake history. But serpent venom and its remedies, so far as lies within my province to discuss them, come also in a special chapter. In concluding this one, I will roughly enumerate the species of rattlesnakes now best known. We have seen that formerly only one or two different kinds were noticed, and the subsequent multiplication of species is due almost as much to science and to a more careful observation of the distinguishing features, as to the discovery of absolutely new ones. The frequent Exploring Expeditions fitted out by the United States Government for Geographical Boundaries, Pacific Railroads, Geological Surveys, etc., with always a zoologist on their Staff of Scientific Men, have added much to our knowledge of natural history; and in the Reports and Bulletins of these may be sifted out information in every branch of Science. Thus in _Crotalus_ chronicles, our two original rattlesnakes have increased and are still increasing. In 1831, the late Dr. J. E. Gray, of the British Museum Natural History Department, enumerated six genera and eleven species belonging to America. In 1860, Dr. Weir Mitchell gave about twenty species as belonging to two genera only, and distinguished by their head scales.