CHAPTER IX 



THE CRAYFISH: A STUDY IN GEOGRAPHICAL 

 DISTRIBUTION 



Every animal is adjusted to particular conditions of temper- 

 ature, moisture, light, contact, food, and other factors of 

 environment. These adjustments determine, as has been 

 abundantly illustrated in the preceding chapters, the exact 

 situation an animal shall occupy. The situation is known as 

 its habitat. The number of different kinds of hal)itats is very 

 great. They may, however, be grouped into certain large 

 classes, so that we may distinguish marine animals, fresh- 

 water animals, terrestrial animals, subterrestrial animals, 

 aerial animals, and so on. A particular species lives normally 

 in one of these classes of haliitats, and its distribution on the 

 earth is limited by the bounds of its hal)itat. Thus the dis- 

 tril)ution of the brook trout follows branching lines on the map 

 Ijecause the streams themselves do so. There are very few 

 animals, either in the sea or on the land, whose (hstribution 

 is uljiquitous, just because their .suitable habitats are widely 

 separated. 



But not all of the similar habitats all over the whole world 

 are occupied by a single species adapted to .such a habitat. 

 The trout of the Ijrcjoks of one continent are not exactlj' the 

 same as those of another continent. We find rather, in widely 

 separated areas, different species which occupj' precisely 

 the same kinds of habitats. Consequently the traveller who 

 journeys from his native land to a foreign country meets there 



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