July 14, 1898] 



NATURE 



259 



Although these do not give us the details as seen by the unaided 

 eye, or as photographed on a small scale, they serve to show 

 the structure of the lower corona. It is difficult, however, for 

 reproductions such as these to do justice to the original nega- 

 tives, as much of the fine detail is lost in the process. Prof. 

 Campbell, it will be remembered, was stationed at Jeur, and his 

 chief instrument was a large photographic telescope of S inches 

 aperture and 40 feet focal length, the instrument being fixed, 

 and the photographic plate made to follow the sun. 



THE PLANKTON OF LAKE MEND OTA. 

 'HPHE natural history of small lakes has long offered a most 

 promising field for research in an important department of 

 biology, viz. the inter-relations of species of plants and animals 

 in the struggle for existence, and the dependence of both upon 

 the physical factors of their environment. As compared with 

 the majority of land and sea areas, a small lake constitutes a 

 relatively perfect "unit of environment," the different elements 

 of which can be determined with an accuracy impossible in most 

 other cases. It is on this account, we suppose, that the detailed 

 study of lake plankton has rapidly gained so many votaries 

 since the lines of quantitative investigation were laid down by 

 Dr. Zacharias and his pupils. In America, especially, the in- 

 vestigation of lacustrine plankton has been taken up with zeal 

 by a considerable army of workers, the vast network of lakes in the 

 1 asin of the St. Lawrence and the upper reaches of the Mississippi 

 providing unrivalled opportunities for the most diversified in- 

 quiries. The latest ' contribution upon this subject is at least 

 as interesting as its predecessors, and we propose here to give a 

 short account of Prof. Birge's principal results. 



Lake Mendota is a sheet of water 6 miles in length by 4 in 

 width, of moderately uniform depth, varying from about 18 to 

 24 metres, and without any large affluent. During the winter 

 the lake is usuadly frozen over for three or four months. In the 

 present memoir Prof. Birge gives an account of the Crustacea of 

 the plankton of this lake. He deals firstly with the seasonal 

 and annual changes in the frequency of the Crustacean con- 

 stituents of the fauna, and secondly with the horizontal and 

 vertical distribution of the total Crustacean population and of 

 the individual species. In each case he discusses the nature and 

 influence of the various factors which operate in the production 

 of the observed changes. Serial observations and collections 

 nere made during a period of two and a half years. 



Neglecting isolated individuals, the Crustacean fauna of Lake 

 Mendota consists of eight well-represented species, which may 

 c grouped as (a) perennial and (b) periodic. The perennial 

 <ioup includes three species of Copepoda {Diaptomtis Ore- 

 i^onensis, Cyclops brevispinosus and C. Leuckartii), and two 

 species of Cladocera (Z)a///«/a hyalitta&nd Chydortis spharicus). 

 The periodic group consists entirely of Cladocera (viz. Daph?na 

 pii/ex, D. retrocurva, and Diaphanosorna brachyurum). 



Prof. Birge shows by an elaborate series of curves and figures 

 that the Crustacean population undergoes a cycle of seasonal 

 changes which is regularly repeated in successive years— three 

 periods of increase alternating with three periods of decrease 

 in the course of each year. The maxima occur in spring (May), 

 midsummer (July), and autumn (September and October) ; the 

 minima in winter (December to April), early summer (June or 

 early July), and late summer (late July or August). 



The spring maximum is by far the greatest, and is due mainly 

 to the rapid and preponderating increase of Cj'f/(?/i-(5r<?z'?j//;wj«j. 

 The summer depression is due to a subsequent rapid decline in 

 the numbers of this species. Renewed reproductive activity on 

 the part of other perennial species leads to the midsummer 

 maximum, which is succeeded by a slow decline, reaching a 

 point of greatest depression towards the end of August. During 

 this period of decline most of the periodic species are intro- 

 duced, but their numbers do not, as a rule, compensate for the 

 falling off in the number of the permanent species. In this 

 respect Lake Mendota appears to be peculiar, for it often happens 

 in other lakes that the periodic forms are the dominant members 

 of the summer [wpulation. The September rise is caused 

 chiefly by the multiplication of Daphnia of all species and of 

 Cyclops. The rapidity of the subsequent decline to the winter 



1 "Plankton Studies on Lake Mendota. II. The Crustacea of the 

 Plankton, July T894-December 1896." By E. A. Birge. Ph.D., -ScD.. 

 Profes-sor of Zoology, University of Wisconsin. (Trans. Wisconsin Acad 

 Set., XI., 1897, pp. 274 to 448.) 



NO. 1498, VOL. 58] 



minimum is dependent on a number of different conditions, such 

 as the abundance of the periodic forms present, the rate of fall 

 of temperature, storms, &c. It varies therefore in successive 

 years. But while the absolute number of Crustacea present, and 

 the rapidity of the seasonal changes themselves, vary consider- 

 ably in successive years, it is undoubtedly an interesting fact, 

 clearly established by Prof. Birge's researches, that the general 

 character of the vicissitudes of the floating population of the 

 lake is remarkably constant from year to year. 



The principal factors which determine the numbers of Crustacea 

 in different years are, according to Prof. Birge, (i) food supply, 

 both quantity and quality, (2) temperature, and (3) competition. 

 It would appear that of these factors, the temperature of the water 

 exerts a greater control over the number of Crustacea than does 

 the food, since the number of Crustacea falls off in autumn 

 while food is still abundant. The influence of temperature is 

 felt through its effects upon the reproductive powers of the 

 Crustacea, increased warmth favouring rapid multiplication. 



So far as the food supply of the Crustacea is concerned, Prof. 

 Birge assures us that the actual quantity of microscopic plant- 

 life in Lake Mendota is almost always in excess of the demands 

 of the Crustacea. A scarcity of food is brought about by changes 

 in the quality rather than in the quantity of the algse present, since 

 j some forms are more available than others as food for particular 

 species or stages of Copepods or Cladocera. For example, young 

 Crustacea are quite unable to eat Ceratium on account of its 

 large size and its hard shell ; consequently the regular pre- 

 dominance of Ceratium in the late summer is one of the principal 

 causes which brings about the annual decline in the number of 

 Crustacea at this season of the year. The Cladoceran Chydorus 

 remains scarce while diatoms or Ceratium are the predominant 

 algje, but abounds when the place of these algie is taken by 

 Schizophyceoe or Anabatia. In seasons when the inedible 

 filaments of Lytigbya predominate, there is a marked reduction 

 in the numbers of all Crustacea present, except Diaptomus, which 

 manages to maintain its numbers by combining great locomotive 

 powers with effective means of catching food. 



Equally interesting is Prof. Birge's account of the vertical 

 distribution of Crustacea in the lake at different seasons. In 

 winter, corresponding with the homothermous condition of the 

 water, the Crustacea are uniformly distributed ; but in summer 

 the formation of the "thermocline " (or boundary between the 

 upper stratum of warm, and the lower stratum of cold water) 

 leads to a distinct stratification of the lake into layers inhabited 

 by different types. The layers undergo changes in thickness as 

 the thermocline descends, and these changes affect the dis- 

 tribution of the Crustacea to a marked degree. Moreover the 

 layer of cold water below the thermocline becomes largely 

 exhausted of oxygen by the decomposition of dead plants and . 

 animals which sink into this stagnant zone ; and it is on this 

 account, rather than on account of the difference in temperature, 

 that the layer below the thermocline becomes largely destitute ot 

 Crustacean life. Insect larva;, however, such as Corethra, 

 may nevertheless be found in considerable number below the 

 thermocline, obviously because they can carry a stock of air in 

 their breathing tubes. 



Space will not admit of further references, but we have per- 

 haps extracted enough from this excellent memoir to justify our 

 opening remark that the careful study of lake plankton is well 

 worth the expenditure of time and labour such as the author of 

 the memoir before us has clearly devoted to it. W. G. 



DESTRUCTION OF THE FRENCH OBSERV- 

 ATORY IN MADAGASCAR. 

 A N interesting account of the destruction of the French 

 Observatory in Madagascar is contributed by M. E. Colin 

 to a recent number of Cosmos. 



In October 1895, after the rupture between the Governments 

 of France and Madagascar, the colonists and missionaries of 

 the former country were requested to leave Antananarivo. 

 The observatory of Ambohidempona, belonging to the French 

 Catholic Mission, was entrusted to the care of the Prime , 

 Minister by the priest Mgr. Cazet, together with all the instru- 

 ments. The two natives, who acted as computers, were instructed 

 to continue the series of observations commenced in 1889. 

 Matters went well and quietly for a time ; but after about nine 

 months had elapsed a rumour was circulated by an Indian, a 

 British subject, to the effect that the French before leaving had 



