Why did early investigators predict that there must be a reduction division in the sexual reproduction process?

Chromosomes come in pairs (2n) in somatic cells or cells of the body. For instance, the number of chromosomes of a human being is 46 (23 pairs of chromosomes). In the event of sexual reproduction, gametes (sperm and egg cells) contain the same number of chromosomes as somatic cells then the union of the sperm and egg will produce twice that of normal (4n) chromosomes and will render reproduction inconsistent or even impossible. Therefore, a conclusion is made that in the cellular division of gametes, the chromosomes should be reduced in half in order to produce the same number of chromosomes for somatic cells generation by generation. 

The infrared (ir) and the ultraviolet(uv) represent the two extremities of the visible spectrum (400-700nm). while ir represents electromagnetic radiation with wavelengths longer than those of visible light, uv represents wavelengths shorter than visible light