Rats! Who said rats? That is an important question, because the word means different things to different people. To some persons “rats” means simply “nonsense”! To Sir James Crichton Browne it means the devastator of stores and the dread carrier of bubonic plague. To the naturalist it means a group or natural cohort of small mammals similar to our common rat and mouse, representatives of which are found in every quarter of the globe and in almost every island of the sea. The distinct “kinds” or “species” are numbered by the hundred. They are extraordinarily alike, and can only be distinguished and classified into proper “species” by careful examination and measurement. Mr. Oldfield Thomas, of the Natural History Museum, has made a special study of them. To give an idea of his work, it may be mentioned that ninety different names had been given by previous writers to as many apparently distinct kinds of rat occurring in India. But by careful measurement and study of the relations to one another of these rats, Mr. Thomas has reduced the number of really distinct Indian species of rats and mice (for a mouse is only a smaller rat) to nineteen. What we call in English water-rats, or water-voles, field-voles, and such little foreign beasts as the lemming and the hamster, are very close to rats in appearance, but are separated on account of clear differences of structure from true rats and mice.

At a meeting in London the total destruction of “rats” was advocated. Whether it was affirmed at the meeting, or was merely an error of those who wrote and commented on the matter afterwards, I do not know, but it was very generally stated in this connection that the old Black rat (known to naturalists as Mus rattus) is quite extinct in England, and that its place has been taken by the Norwegian, or Grey rat (Mus decumanus), also called the Hanoverian rat, because it became noticeable by its abundance in this country at the time of the accession of the Hanoverian kings. The Black rat is not extinct in England, not even very rare. Mr. Stendall lately sent me specimens caught in his warehouse in the City of London, where they are abundant. In many localities, e.g. Great Yarmouth, and in isolated dwelling-places they occur, and even outnumber the Norwegian rat. A most important and remarkable fact is that the rats which infest ships are often all Black rats. The Black rat, or Alexandrine rat (as Mr. Thomas calls it), lives in our houses, in the roof, in recesses of woodwork. It is a house rat, whereas the Grey, or Norwegian rat, lives in the sewers and the banks of ditches, and only comes up into the basement of houses through defective building. The Grey rat has driven out the water-voles from many river banks near towns, just as he has to a great extent taken the place of the Black rat in houses where the kitchen and food stores are close to and in communication with the sewer!

The Black rat cannot be really distinguished by his blackness. That is why some naturalists call him the Alexandrine rat, so as to avoid a misleading implication. He is often of a bright yellowish-brown colour along the back—with longer dark-brown hairs and a good deal of grey elsewhere—quite like the Norwegian or Grey rat in colour. At the same time he is often blackish, and frequently very black. The colour of all these kinds of rats and mice can vary, according to the conditions and colour surroundings in which they live. Black, white, sandy-brown, or a mixture of spots of all three colours, or a uniform “mouse-brown” tint, are (as most boys know) the possibilities revealed by allowing them to breed in captivity. Nature selects accordingly the particular tint which affords protection from observation by enemies in a given locality.

The real distinction between the Black (Alexandrine) rat and the Grey (Norwegian) rat is that the Black rat is smaller, has a tail longer than its body (125 per cent.), and long and wide ears, which stand out from the head. The Grey (Norwegian) rat is a larger, heavy-bodied rat, with a tail shorter than its body (90 per cent.), and short ears. Both these rats are common in India, but there is a third kind, which is the commonest of the three in Calcutta, and is probably the one most concerned in the dissemination of plague. It differs in some definite features from both the Black rat and the Grey rat, although it is very much like the latter in general appearance. It is called Nesokia Bengalensis, or Mole-rat. It is a big rat—its tail is only 70 per cent. the length of its body; the pads on the soles of its feet differ from those of the two other rats; its fur is thin and bristly, and when it is put into a cage it erects its bristles and spits! It is, like the Black rat, a stable and granary rat, and makes burrows in which it stores grain.

The rats of Calcutta have been carefully studied lately by Dr. Hossack, in consequence of their connection with the bubonic plague. In the older native parts of Calcutta, the Mole rat is twice as common as the Norwegian Grey rat, and the Black rat not so abundant as the latter. In the central European part of the town the Grey rat is commoner than the Mole rat—because, apparently, the better-built houses do not afford such facilities for burrowing. The Black rat is here also by a good deal the most uncommon of the three. All these rats suffer from the plague, die from it, and the fleas which lived in their fur leave them as they get cold, and make their way on to human beings, whom they consequently infect with the plague bacillus. This has now been quite conclusively proved by the Indian doctors charged by Government with the study of the causes of the plague. The plague bacillus—a minute, rod-like organism, which grows in the blood and lymph, once it has effected a lodgment, and there produces deadly poison—was discovered some fourteen years ago, but it is only recently that the plague bacillus has been shown to live in the intestine of the flea, which sucks it up with the blood or other fluids of the rat on which it lives. The flea, which readily goes to man, does not suffer from the plague bacilli which it has gorged, but conveys them to man either by its bite or by its excrement.

This being so, it becomes important to know all about the fleas of rats. Quite unexpected facts have been discovered in regard to them. In Europe a very large flea is found on the grey and the black rat. This kind has not, I believe, ever been found on human beings or been known to bite them. But in India, in the Philippines, and in the ports of the Mediterranean, this northern rat-flea is rare, and its place is taken by a smaller and more actively vagrant flea, which Mr. Charles Rothschild (who is the great authority on fleas) found upon several different kinds of small animals in Egypt. He named it “Pulex cheopis.” This is the flea (and not our big northern rat-flea) which acts as the carrier of plague-germs from rats to man in India. It appears from experiments that the common flea of man (Pulex irritans) and the cat-and-dog flea (Pulex felis), as well as the big northern rat-flea (Ceratophyllus fasciatus), can harbour the plague-bacillus if fed on plague-stricken animals, but there are no observations to show (as there are about the “Cheops flea”) that they pass habitually from man to rats and rats to men.

It is happily so long (200 years) since we had a real outbreak of plague in Europe that we are still in doubt as to whether the Grey rat or the Black rat is the more susceptible to the disease—and what flea, if any, acts, or has acted, as the carrier from rat to man in this part of the world. The suggestion has been made that the Grey Norwegian rat takes plague less easily than the Black rat, or than the Indian Mole-rat (Nesokia), and that the multiplication of the Grey rat in England and France and consequent decrease in Black rats, is, therefore, an advantage, so far as plague is concerned. Possibly with the Grey rat has come the big rat-flea, which does not attack man as does the Cheops flea. The disappearance of plague in Western Europe seems to correspond in date with the arrival of the Grey rat. But, on the other hand, an alteration in the character of our houses and their greater “accommodation” for the new rat rather than the old black species may account both for the increase of the latter and for the absence of dirt and vermin in the dwelling-rooms and bed-chambers which formerly enabled the plague-bacillus to flourish amongst us, and to reach the human population—as it does now in India and China. All this shows how necessary it is to have accurate true knowledge of such despised creatures as rats and fleas, if we are to live in great crowded cities closely packed together. And it should also make us try to gain further knowledge as to these creatures, so that we may form a reasonable anticipation of the consequences we are bringing down on our heads when we set about exterminating this or that race of animals. We are not yet sure that the Norwegian Grey rat is not a blessing in disguise.