XXV. General laws of fluctuations. 715 Fluctuating variability. Quetelet's law. Individual and partial fluctuations. Linear variability. Influence of nutrition. Periodicity curves. XXVI. Asexual multiplication of extremes. 742 Selection between species and intra-specific selection. Excluding individual [xvii] embryonic variability. Sugar-canes. Flowering cannas. Double lilacs. Other instances. Burbank's method of selection. XXVII. Inconstancy of improved races 770 Larger variability in the case of propagation by seed, progression and regression after a single selection, and after repeated selections. Selection experiments with corn. Advantages and effect of repeated selection. XXVIII. Artificial and natural selection. 798 Conclusions. Specific and intra-specific selection. Natural selection in the field. Acclimatization. Improvement-selection of sugar-beets by various methods. Rye. Hereditary percentage and centgener power as marks by which intraspecific selection may be guided. Index 827 [1] A. INTRODUCTION LECTURE I DESCENT: THEORIES OF EVOLUTION AND METHODS OF INVESTIGATION Newton convinced his contemporaries that natural laws rule the whole universe. Lyell showed, by his principle of slow and gradual evolution, that natural laws have reigned since the beginning of time. To Darwin we owe the almost universal acceptance of the theory of descent. This doctrine is one of the most noted landmarks in the advance of science. It teaches the validity of natural laws of life in its broadest sense, and crowns the philosophy founded by Newton and Lyell. Lamarck proposed the hypothesis of a common origin of all living beings and this ingenious and thoroughly philosophical conception was warmly welcomed by his partisans, but was not widely accepted owing to lack of supporting evidence. To Darwin was reserved the task of [2] bringing the theory of common descent to its present high rank in scientific and social philosophy. Two main features in his work have contributed to this early and unexpected victory. One of them is the almost unlimited amount of comparative evidence, the other is his demonstration of the possibility of a physiological explanation of the process of descent itself.