Why natural selection is not just a theory?

In science, the meaning of "theory" is different than the one we use commonly.

In the day-to-day, "theory" means a supposition or speculation, while a scientific theory is an explanation of a certain aspect of nature that is supported by evidence collected from several sources, and that has been tested repeatedly following the scientific method. A scientific theory is considered to be a truth, which is the case for natural selection and evolution.

answer; the tunica intima also called tunica interna, consists of elastic fibers,connective tissue and smooth muscle cells;

igneous rocks are formed by solidification of cooled magma (molten rock), with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks.  

the composition depends on the source magma. there are almost a thousand types of composition found till now.  

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