that here was a fine new field for investigation." His crucial tests only gave him confidence that there was more beyond; then he began the long series of observations and experiments which resulted in the charming volume on "Insectivorous Plants."

It is told elsewhere how he was deterred by theoretical considerations from experimenting on the effects of cross- and self-fertilization, and how the expectation of early results was fairly thrust upon him by the difference in size and vigor between crossed and self-fertilized seedlings.1

When once his attention was fixed, he made a preliminary experiment on two plants, with the effects of cross- and self-fertilization as the principal object of investigation. The results corroborated his previous observations, and he was in possession of the principle. Such simple preliminary experiments are interesting, since, if they do not establish a principle fully, they raise up for it a higher degree of probability than any succeeding experiments, and make it possible to work deductively with considerable confidence.

The introduction of the principle of continuity into general scientific thinking has 1 Effects of Cross- and Self-Fertilization, pp. 6-8.

made it a normal intellectual process to look upon exceptional and isolated phenomena as merely extreme instances of much larger groups. One of the marked characteristics of Darwin's work is that he selected such extreme instances, and sought to connect them with the more common facts to which they were related, by proving, or at least suggesting, their derivation from the latter. Wherever it was possible in his experiments, he varied the amount of a cause in order to note the proportionate variation in the amount of the effect; and where he had to depend upon observation alone, he made strenuous efforts to connect extreme instances by gradations of character.

Thus, he and his son Francis, by continuous attention to the sleeping movements of plants, were able to show that it is not true, as is generally supposed, that the leaves move only in the evening when going to sleep, and in the morning when awaking; for they found no exception to the rule that leaves which sleep continue to move during the whole twenty-four hours, only moving more quickly when going to sleep and awaking than at other times.1 They were able to show that sleeping movements are only highly specialized and exaggerated modifica

1 Power of Movement in Plants, p. 403.

tions of the universal movement of circumnutation. In his experiments on insectivorous plants with phosphate of ammonia he varied the proportion of the latter to determine how small an amount would affect the tentacles of Drosera.1 He found that excessively minute quantities of the latter would produce reaction. His results were so astonishing that in 1873 he doubted his own experiments of 1872, and in 1874 he again thought that some mistake must have been made, and again repeated the experiments, but always with the same results. He discussed these remarkable facts at some length, tried to make them more credible by comparing them with similar cases that are equally astonishing but are known to be true; and expressed the hope that his experiments would be repeated, at the same time laying down the conditions of success.

While studying the power of circumnutation in plants the Darwins accidentally left some of their specimens in several cases exposed to oblique light. Before they "knew how greatly ordinary circumnutation was modified by a lateral light, some seedling oats, with rather old and therefore not highly sensitive cotyledons, were placed in front of a northwest win

1 Insectivorous Plants, pp. 154-173..

dow, towards which they bent all day in a strongly zigzag course. On the following day they continued to bend in the same direction, but zigzagged much less. The sky, however, became, between 12: 40 and 2:35 P. M., overcast with extraordinarily dark thunder-clouds, and it was interesting to note how plainly the cotyledons circumnutated during this interval." These observations they considered of some value from their having made them while they were not attending to heliotropism; and they were led by them "to experiment on several kinds of seedlings, by exposing them to a dim lateral light, so as to observe the gradations between ordinary circumnutation and heliotropism." An accidental observation led to variations in the experiments, which resulted in demonstrating continuity between two apparently distinct classes of movements.

In some of his remarkable studies on gradations of characters, where it was impossible to make experiments, he sought out and observed. Nature's own variations. Perhaps the most striking instance of the study of gradations of character is that connected with the "ocellus " on the tail coverts of the peacock.2 This 1 Power of Movement in Plants, p. 421.

2 Descent of Man, Vol. II. pp. 132-145

feather-mark was properly considered a serious difficulty to Darwin's theory because of its remarkable character. But with consummate ingenuity he undertook to connect it by a series of less and less remarkable markings with the ordinary feather-markings of the group to which the peacock belongs. It is impossible to exaggerate the importance of studying phenomena in their quantitative and qualitative variations, for on it depends the establishment of continuity between phenomena apparently widely separated, and it frequently leads to results that can be reached in no other way.