VOL. I.] DistrihiUion of Land Birds. 229 



J 



and humidity that the floral features of a country (which constitute 

 a bird's most important environment) have been formed; and thus 

 these influences have been the indirect cause in determining the dis- 

 tribution of birds. All forms of life are more or less mutually de- 

 pendent, and consequently whatever has effected the present dis- 

 tribution of plants has also produced the existing relation of birds: 



Passing now from this fundamental cause of the distribution of 

 birds to the secondary factor, let us consider the influence of envi- 

 ronment on distribution. As is well known, there is a very accurate 

 adjustment between the animals and plants of a region, necessarily 

 produced by the struggle for existence and the survival of the fittest. 

 This, in the course of ages, has produced a balance of power which 

 makes animals to some extent necessary to the life of the plants in- 

 habiting the same region, but more especially makes particular 

 plants vitally essential to the life of certain animals. Thus it is 

 that birds, when placed in a new environment, unless it happens to 

 be one especially adapted to their needs, will soon be overcome in 



the struggle for existence. Wallace 



[land 



birds] are adapted to live only in woods, or in marshes, or in des- 

 erts; they need particular kinds of food or a Umited range of tem- 

 perature; and they are adapted to cope only with the special ene- 

 mies or the particular group of competitors among which they have 

 been developed. Such birds as these may pass again and again to 

 a new country, but are never able to establish themselves in it; and 

 it is this organif barrier, as it is termed, rather than any physical 

 barrier, which in many cases determines the presence of a species in 

 one area and its absence from another. While this factor of tem- 

 perature does probably play some part in directly influencing the 

 distribution of birds, I think it is a much less important one than 

 the floral features of the country. Nevertheless, although no longer 

 so vital an operating factor, it is, as above stated, the fundamental 

 or primary cause, and we would accordingly expect to find temper- 

 ature areas for the breeding season to correspond very closely with 

 the distribution of birds; and this, indeed, seems to be the case, al- 

 though sufl&cient data is wanting to prove it. 



We have, then, three factors influencing^ the distribution of birds: 

 first, physical barriers, such as large bodies of water or very high 



Darwinism, pp. ISS'll^ 



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