Marshall however introduced the idea of ‘external’ economies in which the expansion of an industry shifted the average cost curves of individual firms. As a source of external economy, Marshall drew attention to the role of specialisation amongst suppliers, in other words, to variety. The costs of downstream firms are reduced if they have a bigger range of goods to purchase. The argument makes sense if the supplying firms are themselves operating with increasing returns, making for a trade-off between variety and costs.   请问上面的有关内容是Marshall 的那个章节的东东？想找来仔细看看，却觉得无从下手了。   谢谢。