the parallel scratches, they would certainly not have been deeply impressed by them, because they become impressive only when their relation to one another is understood, and this could only be when the glacial theory had been imported from a glacier country.

The importance of the discovery of the theory of natural selection to the work of Darwin's life will be dwelt upon later. A sigh of relief is embodied in the declaration, "Here, then, I had at last got a theory by which to work.” 1 Facts cannot be seen without some notion of the relation they will bear to each other when

they are found. The stupendous importance

of theory for observation is illustrated by the effect of Darwin's theories on biological investigation in all its phases. Huxley put it thus: "The 'Origin' provided us with the working hypothesis we sought."2 The whole biological world was waiting for it; and when it came it carried the biological sciences into the deductive stage, and opened an era of investigation unprecedented in the rapidity with which discovery advanced, and in the accuracy of the results reached.

There are scattered throughout Darwin's

1 Life and Letters, Vol. I. p. 68.

2 Ibid., p. 551.

works numerous illustrations of the importance of theory in the investigation even of matters of detail. Writing of the trimorphic Lythrum salicaria, he said, "The existence of the three forms was first observed by Vaucher, and subsequently by Wirtgen; but these botanists, not being guided by any theory or even suspicion of their functional differences, did not perceive some of the most curious points of difference in their structure. "'1 MM. Boitard and Corbié, in their study of pigeons, had seen and recorded many facts which they could not use, simply from lack of a theory. They had stated that when they crossed certain breeds of pigeons, birds colored like the Columba livia, or the common dove-cot, were almost invariably produced.2 Darwin gave significance to these facts and many others by the theory of descent.

In spite of his unusual power of seeing facts apparently unconnected with the subject under investigation, and his persistent habit of recording results, whatever they might be, Darwin himself, sometimes "not foreseeing the result, did not keep a memorandum of all the facts," which would afterwards have proved useful.

1 Different Forms of Flowers on Plants of the same Species, p. 138.

2 Variation of Animals and Plants under Domestication, Vol. II. p. 14.

No one was more thoroughly convinced of the necessity of clear-cut theory for accurate observation; and he frequently expressed himself to that effect. His son says of him, "He often. said that no one could be a good observer unless he was an active theorizer." 1 He said himself, “I am a firm believer that without speculation there is no good and original observation." 2 "It is an old and firm conviction of mine that the naturalists who accumulate facts and make many partial generalizations are the real benefactors of science. Those who merely accumulate facts I cannot very much respect." 8

1 Life and Letters, Vol. I. p. 126.

2 Ibid., p. 465.

8 Ibid., Vol. II. p. 21.

A

VERY few negative instances have been given to show the importance of theory for accurate observation. They illustrate how the absence of theory led to the oversight or neglect of facts which later, under the sway of theory, have become important. Darwin's works are full of instances in which he was led by his theory to anticipate the facts of nature. It was inevitable that, having so early discovered the theories which covered the whole territory in which he worked, he should be guided by them in the search for facts, and that his work should thenceforth be deductive in its character. Examples of this characteristic method range from the great deductions which led to nearly all his important special investigations, and which illustrate the sweeping consequences of his general theories, to the little deductive details which show how swift and accurate his prevision became even in the matter of minute consequences of these theories.

The minor instances will be given first, and will be followed by the more general ones. Then will follow deductions which he made, but which are still unverified, or have been verified by others; and lastly will be given some of the instances in which he went clearly wrong in his deductions.

The instance about to be given may well be placed first, for the purpose of raising a mooted question in logic. Mill took the position that typically the process of inference consists in reasoning directly from one particular case to another; whereas the older and more generally accepted view is that inference must pass by induction from particulars to a general law, and then by deduction from the general law to other particulars. Darwin had found silicified wood in certain tufaceous formations in Patagonia and on the island of Chiloe on the west coast of South America. He afterwards crossed the Andean Cordillera in an east and west direction, and again found tufaceous formations. In his description of the geological section of the Uspallata range he said: "Many of these tufaceous beds resemble, with the exception of being more indurated, the upper beds of the great Patagonian Tertiary formation, especially those variously colored layers high up the river