128. It would, however, appear that although man is capable of subsisting upon almost every variety of food, he cannot bear with impunity a sudden and abrupt transition from one species to another. This fact was very strikingly exemplified in the eastern part of France in the year 1817, where the failure of the crops occasioned such a famine that the poor were compelled to feed upon such vegetable productions as could be obtained; the consequences are stated to have been general anasarca, interruption of the menstrual discharge, and a permanent loss of strength; the sudden return even to barley bread, after this miserable regimen, was not unattended with danger. Avicenna remarks, that after fasting people should live sparingly; and he tells us that after the famine which occurred in the city of Bochara, many of those who had lived on roots and herbs, and retained their health, became diseased as soon as they returned to a full diet of bread and flesh. A more modern illustration of this principle is to be found in the history of the celebrated but disastrous retreat of Sir John Moore's army, in which many of those who had braved its hardships and privations became victims to the liberality with which they were regaled on a return to their native shores.

Spallanzani has long since shown that animals may be brought to live on food of the most opposite kind, provided always that the change be gradually accomplished; such, for example, as a pigeon on flesh, an eagle on bread, etc. We know that in certain maritime districts of the east, horses are not unfrequently fed on fish, while on the coast of Coromandel they are fattened upon balls of boiled flesh mixed with grain; and it is on record, that a lamb kept on ship-board was fed with flesh until it actually refused the grass when turned into a meadow1. 129. As every description of food, whether derived from the animal or vegetable kingdom, is converted into blood, it may be inferred that the ultimate effect of all aliments must be virtually the same; and that the several species can only differ from each other in the quantity and concentration of nutriment they afford, in the comparative degree of stimulus they impart to the organs through which they pass, and in the proportion of vital energy they require for their assimilation.

Were the degree of excitement which attends the digestion of a meal commensurate with the labour imposed upon the organs which perform it, less irritation and heat would attend the digestion of animal than of vegetable food; for, in the one case, the aliment already possesses a composition analogous to that of the structure which it is designed to supply, and requires little more than division and depuration; whereas, in the other, a complicated series of decompositions and recompositions must be effected before the matter can be annualized, or assimilated to the body. But the digestive fever, if we may be allowed the use of that expression, and the complexity of the alimentary changes, would appear, in every case, to bear an inverse relation to each other. This must depend upon the fact of animal food affording a more highly animalized chyle, or a greater proportion of that principle which is essentially nutritive, as well as upon the immediate stimulus which the alimentary nerves receive from its contact.

In hot countries, therefore, or during the heats of summer, we are instinctively led to prefer vegetable food; and we accordingly find that the inhabitants of tropical climates select a diet of this description; the Bramins in India, and the people of the Canary Islands, Brazils, etc, live almost entirely on herbage, grains, and roots, whilst those of the north use little besides animal food. On account of the superior nutritive power of animal matter, it is equally evident that the degree of bodily exertion or exercise, sustained by an individual should not be overlooked in an attempt to adjust the proportion in which animal and vegetable food should be mixed. Persons of sedentary habits are oppressed, and ultimately become diseased, from the excess of nutriment which a full diet of animal food will occasion; such a condition is best corrected by acescent vegetables. It is well known that artisans and labourers, in the confined manufactories of large towns, suffer prodigiously in their health whenever a failure occurs in the crops of common fruits; this fact was remarkably striking in the years 1804 and 180o. Growing1 youths generally thrive upon a generous diet of animal food; the excess of nutritive matter is consumed in the developement of the body, and, if properly digested, imparts strength without repletion.

Aduts and old persons comparatively require but a small proportion of aliment, unless the nutritive movement be accelerated by violent exercise and hard labour.

1 The reader may perhaps smile when, in support of this general truth, he is led to examine the strata of our globe; when he is told that amidst the ruins of a former world, he will find ample evidence of the fatal consequences of a sudden change in the habits and diet of animals. Mr. Mantell, of Brighton, who has added to his reputation as a medical practitioner that of an original geological discoverer, has observed that, from an examination of the deposit in the chalk of the South-downs, it is clearly demonstrable, that after the catastrophe which broke through the chalk hills, and thus formed their transverse valleys, the basins of the chalk were filled with salt water; the currents of fresh water flowing from the interior, brought down clay, silt, and decaying vegetables, and soon occasioned an intermixture of lacustral testacea; and at length so far changed the nature of the element, as to render it alone fit for the habitation of fresh-water shell-fish. This conclusion results from the exclusive occurrence of marine shells in the lower beds, and that of freshwater ones in the upper; both species, however, appearing intermixed in the intermediate layers.

Now the experiments of M. Beudant (Ann. de Chim. ii. 32) have proved that if fresh-water mollusca be suddenly introduced into sea water, they die in a very short time; whereas, if the fresh water be gradually impregnated with salt, they will even live in it, when of the strength of sea water, without sustaining any injury. Similar experiments on fresh-water mollusca gave corresponding results. I have introduced these facts to the notice of the physiologist, because they establish a uniform law, operating alike upon all living beings, from the insect to man; principally, however, for the purpose of showing that branches of science, however unconnected with, and alien to, each other, may nevertheless be brought to interchange their mutual lights.

130. Those who advocate the exclusive value of animal food, and deny the utility of its admixture with vegetable matter, adduce in proof of their system the rude health and Herculean strength of our hardy ancestors. The British aborigines, when first visited by the Romans, certainly do not appear to have been conversant with the cultivation of the ground, and, according to the early writers, Caesar, Strabo, Diodorus Siculus, and others, their principal subsistence was on flesh and milk; but before any valid conclusion can be deduced from this circumstance, the habits of the people must be compared with those of their descendants. The history of later times will furnish us with a satisfactory answer to those who deny the necessity of vegetable aliment. We learn from the London bills, that scurvy raged to such an excess in the seventeenth century as to have occasioned a very great mortality: at this period the art of gardening had not long been introduced. It appears that the most common articles of the kitchen garden, such as cabbages, were not cultivated in England until the reign of Catherine of Arragon; indeed, we are told that this queen could not procure a salad until a gardener was sent for from the Netherlands to raise it.

Since the change thus happily introduced into our diet, the ravages of the scurvy have been less severely experienced.

1 The aliment of almost every animal, in its first stage of life, is composed of animal matter; even graminivorous birds are nourished by the yolk for several days after being batched.

131. It follows, then, that in our climate a diet of animal food cannot, with safety, be exclusively employed. It is too highly stimulant; the springs of life are urged on too fast; and disease necessarily follows. There may, nevertheless,, exist certain states of the system which require such a preternatural stimulus; and the physician may, therefore, confine his patient to an animal regimen with as much propriety as he would prescribe opium, or any other remedy. By a parity of reasoning, the exclusive use of vegetable food may be shown to be inconsistent with the acknowledged principles of dietetics, and to be incapable of conveying a nourishment sufficiently stimulating for the active exertions which belong to our present civilized condition 1. At the same time it must be allowed, that an adherence to vegetable diet is usually productive of far less evil than that which follows the use of an exclusively animal regimen. I must not quit this subject without adverting to the popular error by which vegetables were denounced as most dangerous articles of food, during the late prevalence of malignant cholera.

The great safeguard in all epidemics, is a healthy condition of the digestive organs, which the sudden abstraction of all vegetable food is by no means calculated to promote.

1 The following is an extract from the letter of a highly intelligent gentleman, lately written during a tour through France: - " The quantity of work done by a French labourer is about equal to that done by an English one; but then he is much longer about it, commencing work at sunrise the year round, and leaving off, perhaps, two hours later than ours. This inability of the French to do the same quantity of work in a given time as the English, is universally ascribed to the meagreness of their diet. The peasantry eat little meat, from habit rather than from inability to afford it. A gentleman compels his own men to do so, and gives them wine (which the peasantry do not usually drink): he says that this system pays well in the increased quantity of work they perform. A neighbour of his, an Englishman, has some English labourers, who live in the English manner, eating much meat, and who do more work ill a given time than their fellow French labourers.

"Generally speaking all the evidence and opinions I have heard tend to establish the immediate connexion between the goodness of the diet, and the quantity of work done".