Two different species may not differ in anatomical structure, may be morphologically similar, but in nature they will not interbreed due to the presence of different chromosome sets. So, under the name “black rat” there are two double species: rats with 38 and 49 chromosomes in the karyotype; under the name “malaria mosquito” there are 6 apparently indistinguishable twin species; tweezers have 3 types of double.
Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.