430 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1961 



I shall give some examples in which work in the Tropics has given 

 results that seem to me of interest to the general biologist, choosing 

 examples in which the results are not such as might be expected from 

 knowledge of the biology of temperate climates. I shall not deal with 

 the economic importance of tropical biology, which is being discussed 

 elsewhere at this meeting. Any economic use of a fauna or flora must 

 be based on knowledge of the general biology; we need not fear, I 

 think, that our results will be valueless even from the economic point 

 of view. 



Before I go on to my examples of tropical biology I must sum- 

 marize the climatic conditions in which tropical and temperate en- 

 vironments differ, insofar as they seem to be biologically important, 

 for it is necessary to realize the nature of these differences if we are 

 to discuss their effects on the animals. I shall speak only of ter- 

 restrial and fresh-water environments, saying nothing of the sea 

 where the differences are of kinds other than those I shall be de- 

 scribing. They would need a separate discussion. 



Ultimately, most of the climatic differences between tropical and 

 temperate regions derive from the greater altitude of the sun at mid- 

 day in the Tropics and from the world distribution of temperature 

 and pressure, which is itself due partly to differences of solar heat at 

 different latitudes and partly to rotational effects. How these ulti- 

 mate causes produce their effects is the concern of meteorology and 

 we need not go into it ; we need only to know what the effects are. 



On the Equator the altitude of the sun at midday is never more 

 than 231/^° from the vertical. It declines to this angle at the solstices 

 in June and December, and is vertical at the equinoxes. In the equa- 

 torial region, therefore, any seasonal change there may be is double, 

 as the sun passes north and south from the vertical. In temperature, 

 however, the seasonal changes are very small, since, with the sun 

 never far from the vertical, the amount of solar heat received does 

 not vary by more than 8 percent, and the variation in the length of 

 the day is insignificant. At the Tropics (23i/2° N. and S.) the 

 seasonal changes are greater, for the sun at the winter solstice is at a 

 height of only 43°, giving a variation of solar heat of about 27 per- 

 cent, and the length of day varies by about 2 hours. In our country 

 [England] sunlight is about twice as powerful in June as in Decem- 

 ber, and the difference in heat received is greatly increased by the 

 much longer daylight in summer. 



As the result of these conditions, the seasonal change in mean tem- 

 perature on the Equator is not usually greater than 1° to 2° C. and 

 is much less than the diurnal range which is often 10° C. In equa- 

 torial regions, however, temperatures are never very high. The an- 

 nual mean is usually between 25° and 30° C, being prevented from 



