Living forms are little affected by average temperature, rainfall, or hunidity, but they are vitally concerned with the actual conditions. Hence climatic extremes are more illuminating to the ecologist than averages. In respect to the climate of the tropics, most of the data available are averages. Some capable ecologists have been deceived by assuming that, because the climate of the tropics is fairly uniform in respect to temperature, it is likewise uniform in regard to rainfall, humidity, wind, and other aspects of climate. Field studies in Jamaica, Hawaii, Fiji, tropical Australia, the Philippines, and South China, together with critical examination of official meteorological records concerning these and other areas, has convinced me that it is worth while to emphasize the climatic variability in the tropics. In addition to the variability from year to year, and from season to season, there is also a notable local differentiation in climate which is of great significance to the ecologist, and hence is also worth pointing out. Although there is comparatively little seasonal contrast in temperature in the tropics, yet near the borders of this zone it is appreciable. At Hongkong (lat. 220), for example, the normal differences between summer and winter is only 2? F. less than at London. Calcutta has a greater range between the three warmest and three coolest months than has Dublin, and Brisbane and Cairo both have twice as much range as San Francisco. Even in regard to extremes of temperature some tropical cities have nearly as great a range as has been officially recorded in the more uniform parts of higher latitudes. For example, compare the extreme recorded range of 640 F. at Calcutta with 740 at Dublin; 520 at Rio de Janeiro with 58? at Wellington, New Zealand; 950 at New Orleans with 860 at Amsterdam; and 7Ij at Johannesburg with go9 at Sitka.1 Although the seasonal range averages less in low latitudes than in high, 1 For many more data illustrating temperature variability in low latitudes, including comparisons with higher latitudes, see Visher, S. S. Variability vs. Uniformity in the Tropics, Scientific Monthly, Vol. I5, pp. 23-35, July, IQ22. [Ecology, Vol. III, No. 4 (pp. 269-351) was issued December i6, I922.]