Before the Second World War agrarian history was invariably treated either as a legal or as a technically agricultural study. The agricultural line of investigation generally confines itself to the history of crops, crop rotation systems, breeds of cattle or agricultural implements and machines. The economic and social evolution of rural, pre-industrial society and even the technical development of agriculture cannot be understood without a knowledge of the history of prices and population. During the period from 1500 to 1800, almost everywhere in Europe more than half the working population was still employed in agriculture. External factors which might seriously affect agrarian production include weather conditions and plant and animal diseases. The relationship between plant growth, and weather conditions is more complicated than is generally assumed in historical literature. There are three factors of importance to the growth of plants, such as temperature, precipitation and intensity of light.