Such a proof, however, can by no means be produced so as to enforce conviction; for it would involve the impossible assumption, either that there is no essential difference between the degree of civilisation of the European nations, in the most ancient and in modern times, or that detrimental circumstances, which have yielded only to the civilisation of human society and the regular cultivation of countries, could not formerly keep up the glandular plague.
The plague was, however, known in Europe before nations were united by the bonds of commerce and social intercourse; hence there is ground for supposing that it sprang up spontaneously, in consequence of the rude manner of living and the uncultivated state of the earth, influences which peculiarly favour the origin of severe diseases.

Now we need not go back to the earlier centuries, for the fourteenth itself, before it had half expired, was visited by five or six pestilences.
If, therefore, we consider the peculiar property of the plague, that in countries which it has once visited it remains for a long time in a milder form, and that the epidemic influences of 1342, when it had appeared for the last time, were particularly favourable to its unperceived continuance, till 1348, we come to the notion that in this eventful year also the germs of plague existed in Southern Europe, which might be vivified by atmospherical deteriorations; and that thus, at least in part, the Black Plague may have originated in Europe itself.

The corruption of the atmosphere came from the East; but the disease itself came not upon the wings of the wind, but was only excited and increased by the atmosphere where it had previously existed.
This source of the Black Plague was not, however, the only one; for far more powerful than the excitement of the latent elements of the plague by atmospheric influences was the effect of the contagion communicated from one people to another on the great roads and in the harbours of the Mediterranean.

From China the route of the caravans lay to the north of the Caspian Sea, through Central Asia, to Tauris.

Here ships were ready to take the produce of the East to Constantinople, the capital of commerce, and the medium of connection between Asia, Europe, and Africa.
Other caravans went from India to Asia Minor, and touched at the cities south of the Caspian Sea, and, lastly, from Bagdad through Arabia to Egypt; also the maritime communication on the Red Sea, from India to Arabia and Egypt, was not inconsiderable.