Modifications are differences in living things that are not inherited, but occur within limited bounds under the influence of external factors. Reproduction between members of the same animal or plant species will not give rise to other identical individuals. The differences between them that are not hereditary are known asmodifications-differences that all biological entities exhibit due to external factors, but which still remain within specific boundaries.
Though identical twins have exactly the same hereditary material, they never resemble one another completely, because it is impossible for environmental conditions to affect them both to exactly the same degree. The external factors leading to modification in living things include food, temperature, moisture and mechanical effects. But since any impact exists in the body only and not the DNA, it remains limited to the individual in question and cannot be transmitted to offspring.
Darwin had claimed that living things could turn into other living things under the effect of environmental conditions, but Mendel proved experimentally that environmental influences could not change living species and showed that heredity took place only within specific bounds. Darwin's ideas remained a theory based on speculation, rather than on experimental evidence. But Mendel's laws of heredity which is the result of a long and patient study and based on experiment and observation, went down in the history of science. Although they were roughly contemporaries, Mendel's genetic studies were accepted by the scientific world only 35 years after Darwin. That was because the science of genetics, for which Mendel laid the groundwork, totally undermined the assumptions of Darwinism, but for a long time evolutionists refused to admit this.
However, scientific progress, obliged them to accept Mendel's findings, and they came to see making minor modifications to their theories as the only way of overcoming this. (See The Neo-Darwinism Comedy.)