THE ARGUMENTS FPwOM DISTRIBUTION. 401 



And so is it with parallel relations in New Zealand, in South 

 America, and in Europe. 



§ 142. Given, then, that pressure which species exercise 

 on one another, in consequence of the universal overfilling of 

 their respective habitats — given the resulting tendency to 

 thrust themselves into one another's areas, and media, and 

 modes of life, along such lines of least resistance as from 

 time to time are found — given besides the changes in modes 

 of life, hence arising, those other changes which physical 

 alterations of habitats necessitate — given the structural 

 modifications directly or indirectly produced in organisms 

 by modified conditions ; and the facts of distribution in 

 Space and Time are accounted for. That divergence and re- 

 divergence of organic forms, which we saw to be shadowed 

 forth by the truths of classification and the truths of embry- 

 ology, we see to be also shadowed forth by the truths of 

 distribution. If that aptitude to multiply, to spread, to 

 separate, and to differentiate, which the human races have in 

 all times shown, be a tendency common to races in general, 

 as we have ample reason to assume ; then there will result 

 that kind of relation among the species, and genera, and 

 orders, peopling the Earth's surface, which we find exists. 

 Those remarkable identities of type discovered between or- 

 ganisms inhabiting one medium, and strangely-modified or- 

 ganisms inhabiting another medium, are at the same time 

 rendered comprehensible. And the appearances and disap- 

 pearances of species which the geological record shows us, as 

 well as the connexions between successive groups of species 

 from early eras down to our own, cease to be inexplicable. 



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