Whatever may be the help which biology derives from these natural ways of experimenting, its progress could only be a very slow one, if it did not possess besides a powerful method for proceeding which is peculiar to it: _comparison_. It is true that every inductive operation implies comparison. We compare what we observe with other real and possible cases. Again we compare when we are experimenting. But, in the comparative method, properly so called, we do not limit ourselves to bringing two cases together. Comparison bears upon a long sequence of analogous cases, in which the subject is modified by a continual succession of almost insensible gradations.[163] How would the general problems of biology receive a solution without this method? If we consider an organism by itself, the complication of functions and organs is inextricable in it. But, if we compare this organism with those which come nearest to it, and then with others which are near to them and so on, disengaging what they have in common, a simplification is produced. The accessory characteristics disappear by degrees, as we descend in the biological series, and, if we have set ourselves to study a certain function, we can finally determine its relation to its organ. Although it belongs to biology, this method has its analogy in other sciences, and especially in mathematics. It appears to me, says Comte, to present a character similar to that of mathematical analysis, which brings forward, in each sequence of analogous cases “the fundamental portion which is common to all, which portion, before this abstract generalisation, was concealed beneath the secondary specialities of each isolated case.” The comparative method, in a word, is a method for analysing biological continuity. Whether it be a question of an anatomical disposition or of a physiological phenomenon “_the methodical comparison of the regular sequence of the growing differences_ which relate to them will always present the surest and most efficacious means of throwing light upon even the ultimate elements of the proposed question.” We see that Comte had here his conception of the infinitesimal calculus in his mind. Better still, where terms are lacking in the organic series he does not hesitate to suppose them to re-establish continuity. He introduces intermediary “fictitious organisms” hypotheses which some day perhaps palæontology will turn into realities. By means of this method, not only we shall know a far greater number of cases, but, what is of more importance, we shall know each one among them better, “as an inevitable consequence of their being drawn nearer together.” We assume, it is true, that all these various cases present a fundamental similarity accompanied by gradual modifications, which always follow a regular course. But this hypothesis as we have seen, is implied in the very definition of general biology.