can be improved by any method whatever, unless the alterations in the density and the curvature are perfectly simultaneous." This is an entire misconception. If a lens has too short or too long a focus, it may be amended either by an alteration of curvature, or an alteration of density; if the curvature be irregular, and the rays do not converge to a point, then any increased regularity of curvature will be an improvement. So the contraction of the iris and the muscular movements of the eye are neither of them essential to vision, but only improvements which might have been added and perfected at any stage of the construction of the instrument. Thus it does not seem at all impossible for spontaneous variations to have produced all the delicate adjustments of the eye, once given the rudiments of it, in nerves exquisitely sensitive to light and colour; but it does seem certain that it could only be effected with extreme slowness; and the fact that in all three of the primary groups, Mollusca, Annulosa, and Vertebrata, species with well-developed eyes occur so early as in the Silurian period, is certainly a difficulty in view of the strict limits physicists now place to the age of the solar system.
A. R. WALLACE
THE PLANTS OF MIDDLESEX
Flora of Middlesex; a Topographical and Historical Account of the Plants found in the County; with Sketches of its Physical Geography and Climate, and of the Progress of Middlesex Botany during the last Three Centuries. By Henry Trimen, M.B., F.L.S., and W. T. Thiselton Dyer, B.A. Pp. xli.,428. (London: Hardwicke, 1869.)
The first local Flora published in England gives a list of the plants of Hampstead Heath. It was prepared by Thomas Johnson, Apothecary on Snow Hill. Early on the morning of the 1st August, 1629, and accompanied by a few friends, he left London and proceeded on a simpling expedition to Hampstead, by way of Kentish Town and Highgate. A heavy shower arrested their progress for a little, but nothing daunted they made their way into the woods, and then on to the heath. The day's excursion was brought to a close in a country inn at Kentish Town, where the party dined together.
Johnson enumerated seventy-two plants as the result of the day's simpling. In subsequent expeditions he added sixty-nine others, so that in 1632 his Enumeratio Plantarum in Ericeto Hampsted, cresc, contained 141 species. This rare little volume, with its forgotten names-those of Gerarde and Lobel-is the earliest precursor of the Flora of Middlesex. Since its publication, the materials have been gradually accumulating for illustrating the Flora of the Metropolitan district, and one of the most valuable features in the work before us is, that its authors have, with great care and singular success, investigated and expounded all the ancient as well as the more recent plant-lore bearing on the subject.
The bi-nominal system of nomenclature and the Linnean classification introduced, somewhat more than a century ago, a new era into the science of Botany, and relegated to comparative obscurity the older authors. The difficulty of determining the value of their names, and the practice of neglecting all ante-Linnean synonymy, have caused their labours to be set aside. The authors of this volume have made the works of these earlier writers critically certain by the help of the Herbaria of Merrett, Petiver, Plukenet, Ray, and specially of Buddle, and of the manuscripts relating to them now preserved in the British Museum.
We have here given for the first time an authentic biography, so to speak, of the different plants so far as they are connected with Middlesex. The name of the observer who first recorded each plant, the date of the record, and the place where it was observed, are specified; while the chronologically-arranged localities where it has been at different times gathered, enable one frequently to trace its increasing rarity, and in not a few cases the biography terminates with the record of its complete extirpation, Thus Turner, the father of British Botany, first (1562) records Penny-royal (Mentha Pulegium, Linn.) from "beside undsley upon the Heth beside a watery place;" after him Gerarde (1597) tells us that it grew "on the common necre London called Miles ende, whence poore women bring plentic to sell in London Markets;" and then, through Blackstone, to our own time when it was found in plenty beside the lampstead ponds, Init finally disappeared from the county about twenty years ago. London Rocket, which Morison says might have been reaped like a crop of wheat on the ruins near St. Paul's after the Great Fire, was, up to the beginning of the present century, a common plant in Middlesex, but is now completely lost. The history of Cucubalus, from the Isle of Dogs, is much shorter. Known in England only in this locality, it flourished apparently in a wild state for twenty years, until building operations destroyed the habitat about twelve years ago.
Every page of the volume supplies similar interesting details in the history of Middlesex plants. This feature of the work is as novel as it is important. There is abundant evidence that the authors, in addition to their faithful and even loving exposition of the labours of their predecessors, possess a sound critical acquaintance with the species of British plants. Even in this aspect, the volume is not behind the best of our county Floras.
The influence of the geological condition of a district upon the organised bodies connected with it has lately been receiving the attention it deserves. Important con- clusions have resulted from the Government inquiry into the relations subsisting between the diseases of man and the geological structure of the south-eastern corner of England. The connection between the indigenous vegetation and the geology of its habitat is not less interesting, and, when data have been sufficiently accumulated to warrant safe deductions, will yield valuable information. M. Thurmann, in an elaborate essay on the botany of the Jura, has shown that vegetation is influenced by the manner in which the particles of the rocks are combined, rather than by the nature of the materials of which they are composed. He has consequently classified rocks into two great groups, based on their mechanical constitution: the one he calls "Eugéogènes" (plentiful-detritus-yielding) and the other "Dysgéogènes" (sparing-detritus-yielding). The essential differences between the two groups are in respect to their hardness, their power of absorbing and retaining moisture in small masses, their permeability in extensive deposits, and the rate at which they form detritus resulting from the possession of these characteristics. Mr. Baker has applied the conclusions of the French
