Is an evolution of species possible over time?

The modern theory of evolution says that with the elapse of many thousands years, living species can change and give rise to new species; this means that this theory denies that species are fixed and immutable as it appears in everyday life. These changes are anyway  so slow to be perceptible only in a few millennia and then, in similar time intervals, the species can almost always be regarded as fixed and immutable as they appear in everyday life. The immutability of species is therefore a valid but relative concept, as its validity is limited to some centuries or millennia, i.e. periods that exceed the lives of dozens of subsequent generations of men, who will not perceive these changes, neither from experience inherited by forefathers or by direct experience.
The generation of a new species occurs when, among the many changes that happen over time, there is one that impels the intercross with the original species.
The current theory of evolution has its basis on two pillars, two phenomena both necessary for the evolution of species: natural selection and genetic mutation.

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