CHAPTER IX. 



RELATIONS OF ANIMALS TO THEIR SURROUNDINGS. 

 VARIATION, AND THE FACTORS OF ORGANIC EVOLU- 

 TION. 



Variation of Species. — Each species of animal has its 

 habitat, to which it is adapted. Every one, however, who 

 collects sliells, insects, fishes, or birds knows that certain 

 species tend to vary. The common pond-snail in different 

 situations, and even individuals from the same pond, 

 may 25resent many variations (Fig. 84). When any body 

 of water is subjected to unusual conditions, such as sudden 

 changes in temperature, or the nature of the water, or the 

 size of the pond, or the saltuess of the sea, the degree of 

 variation is in certain species very striking. Land-shells 

 become dwarfed when living near the sea, or in elevated 

 jilaces, or in hot springs, or where lime is deficient. 



Our fresh-water mussels are strikingly adapted for differ- 

 ent habitats, such as running or still water, while the 

 thick, often rough shells and hinge teeth of the Uuios were 

 evidently developed, says Simpson, in order that they may 

 live in currents or swift streams. The thin, toothless shell 

 of Anodonta is the result of a life in still water or in lakes 

 and canals. 



Those species of Anodonta and Spha;rium which occur 

 in comparatively deep water in lakes, living beyond 

 the disturbing influence of waves, are very much thinner 

 and lighter in color, as well as larger, than the shore forms 

 (Call). 



All species are not equally variable. Although the 

 greatest range of variation occurs in our domestic animals, 

 yet certain domesticated animals, as the turkey and guinea 

 fowl, do not vary, and the cat varies much less than the dog. 



Wide-ranging, much diffused, and common species vary 

 most, and the species of large genera vary more frequently 

 than genera containing but one or only a few species. 



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