portion of the Old World they are found in Asia Minor on ibexes; in India on chevrotains; and in Ceylon on large bats of the fruit-eating tribe. The problem is further complicated by the fact that the three American species are only found on the South American group of deer, which are separated for anatomical reasons from the North American group; and, while the insects of this genus are absent from the North American moose and wapiti, they are characteristic parasites of the closely-allied deer of Northern Europe and Northern Asia. Such is the problem that has to be explained.

Dr Speiser has advanced a theory which seems satisfactory. He suggests that this genus originated as parasites of deer in the New World. One party, in Pliocene ages, wandered south in company with the South American group of deer. To these its members remained attached; and geographical isolation has produced, as one would expect, an isolated group of species markedly distinct from the rest. The party which attached itself to the North American deer became extinct, but not until some remote ancestors had passed with their hosts by way of Alaska and Behring Straits into the Old World. These Pilgrim Fathers of the parasites, travelling westward like the passengers of the Mayflower,' but from America instead of to it, colonised the Old World. Some of their descendants became parasites of ibexes. In later days others, more adaptable in habits and tastes and dwelling in the most southern part of their new home, moved even to stranger hosts, the chevrotains and the bats. It is again noteworthy that the two species of insects found parasitic on the chevrotains and the bats are closely related forms.

One fact more is needed to make this intricate story complete. Ibexes are goats and they are only distantly related to the deer family, but are much more closely allied to sheep and antelopes. Bearing this in mind, it is exactly what one would expect, that of the two remaining allied genera of these flies which are parasitic on hoofed animals, the one (Melophagus) is found both on sheep and antelopes, while the second (Echestypus) is only found on antelopes and is peculiar to Africa—the head-quarters of the antelope tribe. Nor is it surprising that the familiar parasite of the sheep (M. ovinus) should be

recorded, not only in North and South America, but also in Australia, to which it has travelled with its hosts.

Next come the parasites of the lemurs, which form a distinct genus (Alloboscs) with one species from Madagascar. It is a form which shows such reduction in the size of the wings that the insect's powers of flight must be affected. This is of interest because it is characteristic of many parasitic insects whose hosts have the habit of returning regularly to the same sleeping-quarters. Such is actually the habit of lemurs.

When we turn to parasites of birds, entirely different considerations arise. Moreover, the searcher after ordered generalities is confronted with some fifteen genera and nearly a hundred species of Hippoboscid flies which are parasitic on birds. It will not have escaped the notice of the intelligent reader that the special parasites of the ostrich are nearly allied to the parasites of the camel, and belong to a group which otherwise only infests mammals. Ostriches are abnormal birds and incapable of flight. There are, however, many other birds which have immense powers of sustaining themselves on the wing. Some stir was caused by a species of fly, belonging to the Hippoboscid family and first described from Java, appearing in a collection made at Saumur in France. There was no longer any mystery when it was known that the fly had been taken from a straggling frigate-bird driven by stress of weather from the tropics to Europe. Thus the distribution of the parasite is involved with that of the host.

Certain outstanding features of the species parasitic on birds have been established. They have for the most part a wide geographical range, and are found indifferently on a variety of birds. Birds of prey frequently take on the parasites of their victims; and parasites of small birds are constantly obtained from the nestlings of sparrow-hawks and other Raptores. Certain genera (for instance, Pseudolfersia) show a taste for water-birds; others have a marked preference for swallows or swifts. Of the forms of fly with well-developed wings and effective flight powers, it does not seem that more than two are confined to special hosts; these are the parasites of the ostrich and of the frigate-bird.

We have in the British Islands only three species of these bird-parasites. The first is so catholic in its tastes that it is found on a great variety of birds extremely different from one another-game-birds, owls, woodpeckers, thrushes, and warblers. It is a species with well-developed wings which are retained through life; and, if the host be killed, the flies will follow a person carrying it. The second species is found in or near the nests of house-martins under our eaves. The third infests swifts.* It is established that the European parasites of the swallows and swifts do not accompany them to their winter quarters, and that it is only during the nesting season that these birds are infested.

The forms with reduced wings and poor powers of flight are often attached to special hosts; such are the parasites of the swallows and swifts. It is significant that both these two last-named families of birds, which are not otherwise allied, often nest in colonies. In such a case the loss of the power of flight would not be detrimental to the parasite. It is also worthy of note that the special parasites of those primitive mammals the kangaroos are considered by Dr Speiser to have most affinities with certain bird-parasites and are included by him in a sub-family containing four other genera, all the members of which are parasitic on birds. Even more remarkable is the fact that a bird parasite (Ornithomyia avicularia), with a world-wide distribution, has in New South Wales and Tasmania become parasitic on wallabies. The sickly animals are mostly infested. This may be regarded as another case of adversity bringing strange bed-fellows.

HAROLD RUSSELL.

The first (Ornithomyia avicularia) has been carried by birds all over the globe. The parasite of the house-martin is Stenopteryx hirundinis ; that of the swift is Oxypterum pallidum. The American swifts have their own parasites of a different genus. The swifts (Collocalia) from

whose nests the Chinese make soup have yet another.

Art. 7.-THE IDEA OF PROGRESS.

1. Essai sur l'Histoire de l'Idée de Progrès jusqu'à la Fin du xviiie Siècle. Par Jules Delvaille. Paris: Alcan, 1910. 2. The Idea of Progress. By Prof. J. B. Bury. Macmillan, 1920.

THERE was a day when it was easy for Sir John Seeley to maintain that history was the biography of States, and when it was no less easy for E. A. Freeman to show that it was past politics. That day, however, is more remote than the age before Aug. 4, 1914. It is true that the life of the State must always engross the attention of the historian, but it is no less true that he must pay heed to the ideals by which men live. Voltaire realised the change when he set forth his 'Siècle de Louis XIV,' and his more remarkable Essai sur les Mours.' Now all competent historians agree with Thomas Arnold that history is the 'biography of society.' In this biography thought and action must be the subject of our study.

The civic ideal of the classical world, the monastic ideal of the early Middle Ages, the chivalrous ideal of the later Middle Ages-these are written so plainly on the pages of history that no one can ignore them. Each in turn passes away, and we are interested in the causes of the passing. The inquirer to-day perceives the altered ideal, and he naturally desires to note the trend of events when an ideal is translated into action. In a word, he assumes that there must be progress. The idea of progress, however, is wholly modern, and was inconceivable before the 16th century. Even to-day it is a conception repellent to the Eastern mind. The majority of mankind agree with the attitude of the punkah-puller. An English lady advised him to improve his position. Mem sahib,' he said, when he at last succeeded in grasping her meaning, ' my father pulled a punkah, my grandfather pulled a punkah, all my ancestors for four million ages pulled punkahs, and, before that, the god who founded our caste pulled a punkah over Vishnu.' The thing that hath been is that which shall be, and there is no new thing under the sun.

6

Sir Henry Maine used to complain that men failed to

understand how exceptional were the conditions of Western society, and he pointed out that the progressive conception which underlay them was one of recent growth. It is remarkable that it has not occurred to any writer till 1910 to trace the history of the idea of progress. No doubt there is a discussion in Comte's Cours de philosophie positive,' but, like so many of Comte's ideas, it is only thrown out in passing. There are also notices in the volumes of writers like Buchez, Javary, Rigault, Bouillier, Caro, and Brunetière. All these discussions are, for the most part, fragmentary and too brief to add to our knowledge. It is significant that all these thinkers are French. In this field Germany contributes nothing, a fact which is puzzling when we remember the importance which she attaches to Kulturgeschichte.

In 1910 M. Jules Delvaille produced a massive monograph on the question. He begins with classical times, and comes down to the end of the 18th century. Of his erudition there can be no question, though there can be a question of his power to use his erudition. He has written a history of thought. The point is, Has he written a history of the growth of a particular thought? Here his volume disappoints the reader. He quotes author after author; he has not only read but he has also meditated; yet he seldom helps us to realise how wholly modern is the idea of progress. The information is in his pages; but he is unable to tell us what is important and what is not. It is not enough for a thinker to mention the word progress. Bodin does this, and we know no writer to whom the idea is more repellent. Does the conception fit in with the rest of the scheme of thought of the writer discussed? Were there, above all, conditions at work at the time in which he wrote to render progress possible? To these questions M. Delvaille vouchsafes no answer. Prof. Bury's recent book is free from these defects. It is marked by that insight we have long learnt to associate with his name. He brings his investigation down to the middle of the 19th century, leaving us regretful that he did not reach the year 1920. The outstanding feature of his illuminating volume is the attention paid to the decisive steps in the growth of the conception. In it there is encyclo