404 LAND ANIMALS 



of species; the number of individuals is also very great, and in Java, 

 for example, may equal the total number of birds that occur in the 

 same region. 90 



With such an abundance of species, it is clear that the struggle for 

 existence between animals is extraordinarily intense, in contrast with 

 the slight struggle with inanimate nature. In the tropics, the adapta- 

 tion of these animals for this struggle is therefore unusually stressed. 

 Here we find the number of poisonous animals, and the virulence of 

 their venom, increasing toward the tropics. Scorpions and centipedes, 

 which are more abundant in the tropics, are also more venomous there. 

 Repulsiveness to taste and smell occur more often in tropical than in 

 the temperate belts; and for this reason, together with great inten- 

 sification of the competitive struggle among animals, mimicry, pro- 

 tective coloration, and protective form are widely distributed. The 

 phasmids and mantids and the leaf butterflies are mainly tropical. 



The small number of individuals which characterize most of these 

 species is in direct contrast to the great number of species by which 

 the tropical fauna is distinguished. If one disregards termites and ants, 

 the danaids and heliconid butterflies, and perhaps the millipeds and a 

 small number of ordinary species from certain groups, he would find 

 that most species occur only sporadically. A collector of snakes in 

 Cameroon 91 took 50 species, and of only 2 of these did he obtain as 

 many as 4 individuals. This situation appears to be the same among 

 the birds; Whitehead 92 complains that he never obtained a large num- 

 ber of the well-known Bornean birds, and took only one specimen of 

 many species; and Beebe 93 reports from Venezuela that he seldom 

 found many individuals of a species. There was always present an 

 ever-changing panorama; except when the birds were just nesting, he 

 could never be certain of seeing the same species twice, although he 

 was sure that new forms would be added to the list every day. 



It is difficult to give a satisfactory explanation for the small num- 

 ber of individuals. It has repeatedly been observed that number of 

 species and number of individuals are in inverse proportion (see p. 30) . 

 Therefore, it may well be that the intensity of the struggle for exist- 

 ence, which above all other factors causes a reduction in the number 

 of young animals and allows only a few to attain maturity and repro- 

 duction, is one of the causes. Only a certain amount of life can thrive 

 in one place, and when this amount is distributed over many species, 

 there remains only a small number for each species. Still other rela- 

 tionships occasionally come into consideration. Among the plants, it 

 may happen that only a single specimen of a species of tree, for ex- 

 ample, can be found in a large area in tropical forests. Many plant- 



