Another type of variation in rainfall which is prominent in the tropics is the seasonal. Very few tropical localities receive their rainfall as evenly distributed throughout the year as is common in many parts of middle latitudes. Distinct wet and dry seasons are the rule. The rainy summers and dry winters of India and China are well known. Most of tropical Australia also receives almost no rain for six months and from 15 to 50 inches or more in the other six months. Hawaii and many other places near the margins of the tropics receive much of their rainfall in winter, while still other parts of the tropics have two wet and two dry seasons. In order to compare the monthly variability of rainfall in low and middle latitudes, a planimeter measurement was made of Supan's map of Percentage Range of Mean Monthly Rainfall in Bartholomew's Atlas of Meteorology. This map shows four types. of regions: (1) where the wettest month is less than 10 per cent. rainier than the driest month; (2) where the wettest month is from 10-20 per cent. rainier than the driest; (3) where the range is from 20-30 per cent; and (4) where it is over 30 per cent. Tables 6 and 7 show the approximate area and the percentage of each type by continents. Table 6 concerns middle latitudes (30° to 60°); Table 7 concerns low latitudes (30° N. to 30° S.). It will be seen that low latitudes have over three times as large an area possessing a monthly variability of over 20 per cent. as is the case in mid-latitudes and twice as large a percentage of their total area has this range. The one large area in mid-latitudes having the fourth, the most extreme, type of rainfall variability is the Tibetan Plateau, which has little agricultural value because of its great altitude. Furthermore, the month of least precipitation in mid-latitudes commonly is in the winter when plants require little moisture while the wettest month usually is in summer. On the other hand, the driest month of the tropics is also a hot month, with active evaporation. This unfortunate combination is very hard on plants, and is the reason for the lack of forests in many places having a large annual rainfall. For instance, parts of tropical Australia having over 60 inches of rain a year possess no real forest because several months are extremely dry and hot. In respect to the more uniform rainfall type, where the range between the driest and wettest month is less than ten per cent. mid-latitudes have nearly six times as large an area as low latitudes. This type comprises about 26 per cent. of the total land area of mid-laditudes while it makes up only 3 per cent. of low latitudes. Other interesting comparisons come out on further study of these tables. Why should the lack of marked seasons in respect to temperature be emphasized and the presence of marked seasons of rainfall be largely ignored by most writers on the tropics? Another climatic factor subject to marked changes is storminess. Cyclonic storms are erratic in all parts of the world but the extremes appear to be greatest in low latitudes. The range in the number of hurricanes damaging Australia, for example, has been from one hurricane in 1907 and 1919 to seven in 1916 and eleven in 1912. In Fiji some years have none, but several years have had three each and one year four. In the South Indian Ocean the variation reported by the Mauritius Observatory has been from one storm in 1900 to eight in 1894 (and several other years) and to thirteen in 1913. In the Philippines in a 15-year period the number of very severe typhoons varied from one in 1916 to seven each in 1908 and 1911. In respect to less violent cyclonic storms there appears to be a somewhat similar range. For example, the total number of well-marked tropical cyclones occurring in Queensland, Australia, varied from eight in 1920 to 24 in 1916. In respect to the month of occurrence, as well as in annual frequency, there likewise is marked irregularity. In some years cyclones may be lacking during the months when they normally are most frequent and occur only in months supposed to be VOL. XV-3 free from dangerous storms. Of thunder storms also there is marked variation, perhaps more than in higher latitudes. Many stations in Fiji and elsewhere have experienced several times as many in one year as in another. While many hurricanes are accompanied by appalling lightning, other equally severe hurricanes have none. Slight changes of weather are almost constantly taking place. in the tropics. A rainy spell will be succeeded by a less rainy one or by a few rainless days; a hot spell by a slightly cooler one; a spell of fitful breezes, by several days of steady winds. Such changes have been noticed by the writer in Jamaica, Hawaii, the Philippines, the East Indies, Queensland and elsewhere, but have been especially studied in Fiji. There, a study of the official records taken at Suva reveals an average of about 20 distinct spells of weather well distributed throughout the year, with about as many less distinct changes. In conclusion, when all these types of variation occur, is it right to give the impression that tropical climates are extremely uniform? But although tropical climates are not so uniform as has been supposed, it does not follow that they are better adapted to civilized man than has been supposed. Most of the variability within the tropics is of a highly irregular sort compared with the variability characteristic of the parts of the higher latitudes where civilized man mostly lives. Indeed it appears that tropical climates are unfavorable for a high type of civilization not alone because of the high temperatures and the general lack of stimulating seasonal changes in temperatures, but also because of the often extreme undependability of the rainfall, the occurrence not infrequently of destructive windstorms and other unfavorable variations. But, nevertheless, highly civilized man can cope with the numerous problems of the tropics far better than can primitive people. Indeed, the latter, unaided, have made little progress. Hence fuller utilization of the tropical resources awaits a greater participation by civilized man. THE POSSIBILITIES OF EXTERMINATING INSECTS By Dr. E. P. FELT STATE ENTOMOLOGIST OF NEW YORK HIS is a special phase of the war against insects, the general THIS aspects of which have been discussed in such an illuminating and very suggestive manner by Dr. L. O. Howard, chief of the Federal Bureau of Entomology, in his address as retiring president of the American Association for the Advancement of Science.1 Dr. Howard has given in this account a most excellent summary of the broader phases of insect control, though, for some reason, possibly because he knew of the writer's interest therein, he refrained from discussing exterminative measures or the possibilities of eradicating isolated infestations. This is something of considerable practical importance on account of the fact that more than half of our most injurious insect pests have been introduced from abroad and the process is still going on in spite of the widespread and, as a whole, well directed Horticultural Inspection Service of the general government and the various states. This latter work has undoubtedly prevented the establishment of a number of injurious insects and delayed the introduction of others, though occasionally we find a destructive pest well established in a section and in the ordinary course of events destined to extend its range and possibly inflict very serious losses over a considerable period of years. The Gipsy Moth, the Brown-Tail Moth, the Elm Leaf Beetle, the Leopard Moth and the recently introduced Japanese Beetle are somewhat familiar examples in the eastern United States, while the south has become altogether too familiar with the Boll Weevil, the Pink Boll Worm and very lately the Mexican Bean Beetle. There is, in addition, the recently introduced European Corn Borer, now beyond any possibility of extermination so far as this hemisphere is concerned, though at one time it must have been within possibilities. We are also confronted in the early history of the Gipsy Moth with the futile attempt of the state of Massachusetts to exterminate the insect, while later developments dem 1 (a) "On Some Presidential Addresses''; (b) "The War against Insects."' Science, 54: 641-651, 1921. onstrated beyond question the practicability of accomplishing what at one time appeared to many as an unattainable ideal. It must be conceded at the outset that problems of this nature are surrounded by manifold difficulties and that, within certam limits, each case must be decided upon its merits. In the first place, it is exceedingly difficult to determine the future status of an insect before it has become well established and thus presumably ineradicable, unless some unusual limitation makes extermination relatively easy. Granting that there is substantial agreement among scientific men as to the desirability of exterminating a given species, the great problem of educating the public to view the matter from the right standpoint and thus make possible the securing of means to prosecute a vigorous campaign still remains to be solved. Furthermore, initial operations, if the undertaking is to be successful and conducted in the most economical manner, must ordinarily be started before successful measures have been thoroughly demonstrated. There is always an element of doubt in regard to the possibility of serious injury, the feasibility of extermination and the methods to be employed, consequently it is not easy to secure a combination which will bring about the desired results. On the other hand, there is practical agreement among most scientific men familiar with the work of insects to the effect that extermination, when possible, is immensely cheaper and more desirable than the prosecution of more or less unsatisfactory control measures in a constantly expanding infested territory. Earlier attempts to exterminate insects were based largely on some plan designed to catch or kill the last remaining insect, preferably within a year or two and certainly within a few years. Some have even advocated reducing the infested territory to practically desert conditions in such a manner as to make all insect life at least impossible. This latter is undoubtedly possible in the case of very restricted infestations and may be justified if the insect is an exceedingly destructive or dangerous one. It is out of the question if an extended area is infested or the insect one which is not particularly dangerous to life and does not threaten a basic crop or industry. Most cases come in this latter category and therefore do not justify extreme or drastic measures. It seems to the writer that the method of progressive reduction, if one may use a special term, has not received the consideration it deserves, and yet it has been the method which has brought about extermination of Gipsy Moth colonies in areas well removed from the generally infested territory. The plan in such a case was to bring about conditions unfavorable for the multiplication of the insect and, by following up the matter from year to year, |