ECOLOGICAL FACTORS 399 



light and dark plays a role in the struggle for existence, in so far as it 

 permits many animals to exist beside one another, since some rest at 

 the time when others are active so that the competition between them 

 is reduced. 



On the greater part of the earth's surface there is still another 

 cycle in addition to this short period, namely, the annual seasons. 

 However, there are places where such differences in the weather do not 

 occur, so that one may speak of regions where temperature, humidity. 

 and sunlight are essentially the same on one day as on another. In 

 such regions the concepts of weather and climate coincide. 70 Most of 

 these places lie between the tropics of Cancer and Capricorn in Amer- 

 ica, Africa, India, and Malaysia. There are a few places elsewhere 

 without pronounced periodic cycles, such as Tasmania, the archipelago 

 of Tierra del Fuego, and the western slope of the Cordilleras in Pata- 

 gonia. In such regions plants grow equally well throughout the entire 

 year, the forest is always green, there is no drying of the vegetation, 

 and the animal population is not involved in seasonal changes; just as 

 in the Cuban caves fishes breed throughout the entire year, 71 and in 

 the grottos of Carinthia insects reproduce continually 72 so tadpoles 

 may be found throughout the year in Tasmania, 73 and there, as in the 

 tropics, the course of life goes on without interruption. 



The tropical environment. — Associated with the absence of sea- 

 sonal periodicity, most tropical localities approach optimal environ- 

 mental conditions, high temperatures and humidities, much light, and 

 a great quantity of food. The poikilothermal animals are particularly 

 favored by conditions which approach those obtaining in the bodies 

 of homoiothermal animals. Heat radiation of the latter is low, and the 

 food requirements are correspondingly less; the food requirements of 

 man, for example, are about 20% less in the tropics than in Germany. 74 

 The utilization of vegetable food by invertebrates is much greater than 

 in the temperate climates; in particular, the Orthoptera and the ter- 

 mites, the greediest plant-eaters among the insects, play a far greater 

 role. From this fact arises a series of peculiarities that distinguishes 

 the animal life of the tropics from that of the temperate and cold zones. 

 The poikilothermal animals are in general larger, and. as in the trop- 

 ical ocean, there is an immense number of species. 



The average weight of the tropical insects is decidedly greater than 

 that of insects of the temperate and cold belts. The large forms of 

 Orthoptera, dragonflies, Neuroptera, Hymenoptera, beetles, butterflies, 

 and bugs are heavier in the hot regions; this is just as true in the 

 phylogenetically old groups (locustids, dragonfiies, Fulgoridae) as in 

 the groups of latest origin (acridids, butterflies, and lamellicorns) ; 75 



