References used: The Origin of Species (first edition) by Charles Darwin. Chapter 1.
Cultivated plants and animals generally show more variety as compared to species bred naturally. This is probably because of the fact that domestic plants and animals are raised in conditions that are not as uniform as the natural habitat of their ancestors. Variation in domestic animals and plants is observed to be a continuous and unceasing process. For example, wheat, which is a very old cultivated plant, continues to produce new varieties.
Claim: Conditions of life are not the direct and only cause of this variation
Evidence: Seedlings of the same fruit and offspring from the same litter can show be considerably different from each other even if both the offspring and parents are raised under exactly the same conditions. If the conditions of life were the only cause of variation in organisms, there would have been no variation among the offspring or even if there was variation, all the offspring should have varied in the same manner. This can be used as evidence of the fact that the laws of reproduction, growth and inheritance have a greater impact on the characteristics of an organism than do the conditions of life directly.
Claim: Correlation of growth. If an organism is selectively bred for a certain characteristic, other parts of the structure of that organism are also modified.
Evidence: Cats with blue eyes are always observed to be deaf. Hairless dogs have imperfect teeth. Long limbs are always accompanied by an elongated head. White sheep and pigs are differently affected by vegetable poisons as compared to their domestic counterparts.
Claim: As a general rule, every characteristic in an organism inherited.
Evidence: When in individuals exposed to the same conditions of life as the rest of the population, a very rare deviation such as albinism, for example, appears which is only found in one of several millions of individuals in a population, the same deviation is often also found in that individual’s offspring. By the laws of probability, one is compelled to attribute its reappearance in the offspring to the laws of inheritance.