THE ARGUMENTS FROM DISTRIBUTION. 489 



cannot be otherwise accounted for. And so is it with parallel 

 relations in New England, 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 modi- 

 fications directly or indirectly produced in organisms by modi- 

 fied 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 embryology, we see 

 to be also shadowed forth by the truths of distribution. If 

 that aptitude to multiply, to spread, to separate, and to dif- 

 ferentiate, 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 those kinds of spacial 

 relations and chronological relations among the species, and 

 genera, and orders, peopling the Earth's surface, which we 

 find exist. The remarkable identities of type discovered 

 between organisms inhabiting one medium, and strangely 

 modified organisms inhabiting another medium, are at the 

 same time rendered comprehensible. And the appear- 

 ances and disappearances of species which the geological 

 record shows us, as well as the connexions between succes- 

 sive groups of species from early eras down to our own, cease 

 to be inexplicable. 



