Summarise the third paragraph of the passage.

Directions: Answer the given question based on the following passage:

With Charles Darwin’s popularisation of the concept of evolution, man became interested in his biological origins and affiliations. This interest was centred on man’s phylogenetic origins. Starting with the work on heredity of Gregor Mendel in the 1860s and the recognition of its significance in 1900, interest shifted to the problem of man’s ontogenetic origins. Mendel’s simple ‘laws of heredity’ came to be widely known and were uncritically assumed to apply to all human characteristics. Most physical characteristics, intelligence, and many aspects of personality were thought to operate as simple Mendelian unit characters in their transmission from parent to child. This belief stimulated an interest to eugenic movements that proposed to bring about race betterment through sterilization of the unfit and encouragement of marriage among the superior members of society. Many groups of people organised to promote such programmes. Feeble-mindedness, poverty, crime, and immorality were thought to be largely matters of poor heredity. Several studies of family lines were published that seemed to give considerable support to this general concept.
From 1900 to about 1920, there was an overemphasis on the role of heredity in producing these conditions. The fixity and certainty of hereditary determination were grossly overestimated, and the environmental contribution to intelligence, personality, morality, and crime was either ignored or underestimated. The one fundamental fact of logic—that correlation does not prove causation—was forgotten. Because certain traits follow family lines, it does not follow that they are necessarily inherited. There is a ‘social heredity’ involved in the transmission of many things from generation to generation, and it is easy to confuse its effects with those of biology heredity.
Starting in the 1920s and ‘30s there set in a reaction against the uncritical application of the simple Mendelian concepts to all human characteristics and the resulting disregard of the importance of environmental factors. The extreme hereditarians were accused of being mechanistic in their frame of reference, deterministic in their philosophy, and unscientific in their methods.
Behaviour trends, intellectual level, special talents, and temperament are not inherited as such, but heredity is certainly a factor in determining all of them. Genes affect behaviour indirectly through their influence on the structural development of the individual. Genes affect structural conditions which, in turn, influence function. Such complex things as feeble-mindedness, musical talent, mathematical aptitude, and criminality can hardly be inherited. However, the genes can be crucial factors in determining the development of the nervous system, the sensitivity of the ear, the discriminating ability of the eye, the length, diameter, and nature of the vocal cords, and as such things as the size and shape of the hands. (Sawrey and Telford in Education Psychology)