This section of the book is from the Guide To Hardy Fruits And Ornamentals book, by Thomas Joseph Dwyer, published in 1903.
A quarter of a century ago this was practically unknown and practiced by very few fruit growers, either amateurs or professionals. Today it is considered one of the paramount and first requirements to the production of the choicest fruit grown, either for pleasure or for profit. No practical, thorough, up-to-date grower of fruit would feel that he was giving his trees the best opportunities and advantages to develop their possibilities if he omitted this important work. In fact, he would not think of neglecting this part which is so necessary in order to produce a choice number one grade of marketable fruit, and the kind of fruit that there is always a demand for at remunerative prices, almost regardless of the general market conditions. Then if our aim is to grow the finest fruit for our own use we must of necessity resort to this "thinning of the fruit." It is not as laborious or expensive a work as one would imagine from the first impulse; we have learned this from long years of varied experiences with the different tree fruits. No work in connection with your fruit growing will pay you correspondingly better than this. Apples, peaches, pears and quinces should be "thinned out," or, to be more explicit, the surplus fruit removed from the tree so that no two specimens will be nearer together than five or six inches. Plums and apricot fruit should be four inches apart. This "thinning out" should be done when the fruit is about the size of a white walnut. Peach and plum trees have frequently been killed by this excessive over-bearing of a comparatively inferior and worthless crop of fruit that under proper "thinning" would have matured a profitable crop, besides being preserved for good money-makers for years to come.