ECOLOGICAL FACTORS 401 



others) ; but the warbling of the birds is not limited so much to a 

 definite and relatively brief period as in our latitudes. 



Especially striking is the tremendous wealth of animal life as well 

 as of plant life in tropical regions. The warmth hastens the course of 

 life activities according to simple physicochemical laws, and therewith 

 sexual maturity; the generations follow each other much faster, in an 

 unbroken series. This again presents additional opportunity for varia- 

 tion and mutation, and therewith for the formation of new species. 



The shortening of the developmental period of tropical animals in 

 comparison with our animals is particularly striking in insects. The 

 butterfly Danaus chrysippus, that has only one generation per year in 

 the northern parts of its range, has in the Philippines one generation 

 after another and requires only about 23 days for its development. 

 The pupal stage of the hawk moths Chaerocampa oldenlandiae lasts 

 for 18 to 21 days on Bohol Island in the Philippines, 24-25 days at 

 Manila, and 8 months in Sydney. 77 Whereas the mason bees Chalico- 

 doma muraria requires two years for one generation in northern 

 Europe, in Java Polistes requires a pupal stage of only 12 days 78 ; the 

 larva of the mason bee in southern Germany hatches in about 14 days 

 and an additional period of 4 weeks elapses before it is ready to spin; 

 in Eritrea, the hatching of Eumenes occurs in 2 days, and pupation 

 follows after an additional 5-6 days. 79 The entire development of the 

 beetle Crioceris subpolita is completed in 25-31 days in Java, 3 whereas 

 in Germany the asparagus beetle {Crioceris asparagi) requires a whole 

 year between generations. A more rapid sequence of generations is also 

 observed in mammals. In New Zealand the red deer brings forth young 

 a year earlier than in Germany. 80 Definite estimates for other animal 

 groups are lacking; nevertheless, they certainly form no exception. 

 This great hastening of development insures rapid succession of birth, 

 growth, propagation and death. If an insect has a length of generation 

 of 30 days in the tropics, and lays, let us say, 100 eggs, the descendants 

 of one pair, assuming that the future generations all live to propagate 

 in like manner, amount to 488 trillion individuals at the close of the 

 year, as against 5000 individuals from two annual generations in the 

 temperate latitudes. 



Some figures will help elucidate the abundance of species native 

 to the tropics. There are 727 species of land snails in the Philippines, 

 while in Japan, iy 2 times larger, there are only 193; Cuba has 614 

 species of land mollusks, Jamaica 463, and in contrast, Tasmania, 

 about 6 times as large as Jamaica, has only 80, while all Scandinavia 

 has only 148 species. 81 The superiority of the tropics in number of 

 species of insects is especially noticeable. If we compare the number of 



