When one or several limbs or organs of an animal are called upon to do extra work, Nature immediately tries to strengthen such members by providing extra nourishment. This extra work must not be carried to the point of exhaustion, or the member may become weakened or diseased.
The habit of life and the work performed play important parts in producing change and variation in individuals, even in so short a period as a life-time. Take two brothers of nearly the same age, similar in looks and characteristics, and approaching their majority, and let one lead an easy, sedentary life, largely within doors, while the other exposes himself to the sun, storms and cold, while employed in laborious toil. When these brothers have reached the age of sixty, they will be so unlike as to suggest that they might belong to distinct races. If the fast-stepping trotter spends his life in drawing heavy loads at a slow pace, and is kept fat in order to secure weight, he becomes to all intents and purposes a draft-horse. He may preserve his fine head and his smallish feet, but in weight and movement he will illustrate what marked changes are wrought in a half-score of years of use. Or allow the well-bred dairy cow to nurse her own calf for a half-year, so that she becomes nearly dry at the end of six months, in subsequent years she will fall short of her normal production; whereas, if the calf had been removed and she had been regularly milked, the flow of milk would not only have been more abundant, but it would have continued much longer and the normal flow of milk have been maintained the following year. The family horse, overfed and under-used, soon becomes slow and logy, although at first he was spirited and active.
It can be readily understood that, in order to preserve the inherited characteristics of a breed or an individual, the animals must not only be placed under similar conditions as to food and climate, but must retain similar habits and do similar work to that performed by the ancestors from which they sprung.
It is difficult to discover certainly the character of the ancestors by an inspection of an animal; it is easy to determine the uses to which they have been put and the abuses which they have suffered. Full use, without abuse, and appropriate work tend to produce variation for the better and to preserve and increase the efficiency of specialized qualities, as well as to make them permanent in time and capable of being transmitted to succeeding generations.