ILLUSTRATIONS OF THE LOGIC OF SCIENCE. 605 



a naturalist wishes to study a species, he collects a considerable num- 

 ber of specimens more or less similar. In contemplating them, he 

 observes certain ones which are more or less alike in some particular 

 respect. They all have, for instance, a certain S-shaped marking. 

 He observes that they are not precisely alike, in this respect ; the S 

 has not precisely the same shape, but the differences are such as to 

 lead him to believe that forms could be found intermediate between 

 any two of those he possesses. He, now, finds other forms apparently 

 quite dissimilar say a marking in the form of a C and the question 

 is, whether he can find intermediate ones which will connect these latter 

 with the others. This he often succeeds in doing in cases where it 

 would at first be thought impossible ; whereas, he sometimes finds 

 those which differ, at first glance, much less, to be separated in Nature 

 by the non-occurrence of intermediaries. In this way, he builds up 

 from the study of Nature a new general conception of the character in 

 question. He obtains, for example, an idea of a leaf which includes 

 every part of the flower, and an idea of a vertebra which includes the 

 skull. I surely need not say much to show what a logical engine 

 there is here. It is the essence of the method of the naturalist. How 

 he applies it first to one character, and then to another, and finally 

 obtains a notion of a species of animals, the differences between whose 

 members, however great, are confined within limits, is a matter 

 which does not here concern us. The whole method of classification 

 must be considered later ; but, at present, I only desire to point out 

 that it is by taking advantage of the idea of continuity, or the passage 

 from one form to another by insensible degrees, that the naturalist 

 builds his conceptions. Now, the naturalists are the gi'eat builders 

 of conceptions ; there is no other branch of science where so much of 

 this work is done as in theirs ; and we must, in great measure, take 

 them for our teachers in this important part of logic. And it will be 

 found everywhere that the idea of continuity is a powerful aid to the 

 formation of true and fruitful conceptions. By means of it, the great- 

 est differences are broken down and resolved into differences of degree, 

 and the incessant application of it is of the greatest value in broaden- 

 ing our conceptions. I propose to make a great use of this idea in 

 the present series of papers ; and the particular series of important 

 fallacies, which, arising from a neglect of it, have desolated philoso- 

 phy, must further on be closely studied. At present, I simply call the 

 reader's attention to the utility of this conception. 



In studies of numbers, the idea of continuity is so indispensable, 

 that it is perpetually introduced even where there is no continuity in 

 fact, as where we say that there are in the United States 10.7 in- 

 habitants per square mile, or that in New York 14.72 persons live 

 in the average house. 1 Another example is that law of the distribu- 



1 This mode of thought is so familiarly associated with all exact numerical considera- 

 tion, that the phrase appropriate to it is imitated by shallow writers in order to produce 



