Nevertheless, as we know, it is not man’s intervention in phenomena which constitutes experimenting properly so called. It consists, before all things, in the rational selection of cases, (natural or artificial, it matters little), which are most appropriate for bringing out the law of variation of the phenomenon under observation. Nature gives us such, for illnesses resemble experiments which we can follow through their entire course and to their termination. They are often difficult to interpret, on account of their extreme complexity, but less so, however, than the majority of the experiments which we bring about ourselves. For are they not more or less violent diseases, suddenly produced by our intervention, without our being able to foresee all their indirect and future consequences? It is pathological anatomy which led Bichat to his fine discoveries in histology and in physiology. And to pathology we must join teratology which is, as it were, its prolongation. Here again, nature supplies experiments which we should not know how to institute.[162]
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.
The comparative method will then apply successively to the different parts of an organism, to the different ages of the same organism, and to the different organisms in the animal and vegetable series. It will even apply to embryonic life, Comte clearly formulates von Baer’s law, while making indispensable reservations. The primitive state of the highest organism, he says, must represent, from the anatomical and physiological point of view, the essential characteristics of the complete state which belongs to the more inferior organism, and so on successively “without our being able to find again the exact analogy of each of the principal terms of the inferior organic series in the sole analysis of the various phases of development of each superior organism.” This comparison, so to speak, allows us to realise in the same individual the growing complication of organs and of functions which characterises the whole biological hierarchy. Thus it is particularly “luminous.”[164] Von Baer’s book had appeared in German in 1827. Had Comte known it, it is most probable that, according to his habit, he would have quoted it.
IV.
In order to consider organisms in the regular sequence which allows of comparison, we must first have established the order in which they should be arranged. But, conversely, to establish this order, a knowledge of anatomy and physiology is indispensable. So between these two sciences on the one hand and “biotaxy” on the other there is a strict solidarity. The problem of classification is thus an essential part of general biology. In the natural classification sought after by science, the position assigned to each organism would suffice to define at once the whole of its anatomical and physiological nature, in relation to the organisms which precede and to those which follow.[165] Any natural classification cannot, however, be anything but imperfect. Accustomed as we are to artificial classifications, which admit of absolute and immediate perfection, we are surprised that the same should not be the case in natural classification. But, if the latter is a real science, we must own that, here as elsewhere, we can only reach more or less distant approximations. The co-ordination of living species is a problem like the static or dynamic analysis of a determined organism. Like this analysis, it only allows of solutions which are approached rather than realised.[166]
How, in the first place, must we understand species? Between Cuvier and Lamarck, Comte sides with Cuvier, with this reservation, however, that “our ideas upon this question of capital importance are not yet properly fixed.” Two reasons especially incline him to admit the fixity of of species. Lamarck’s theory is not sufficiently proved: we nowhere see that the milieu exercises the almost boundless influence upon organisms which is attributed to it by Lamarck. Undoubtedly, within certain limits, the exercise induced by external circumstances tends to modify the primitive organisation. But this action of the milieu and this aptitude of the organism are certainly very limited. On the other hand, if we have a choice between the two hypotheses, the interest of science would prompt us to use this liberty in favour of Cuvier. The fixity of species guarantees that the series of organisms will always be composed of terms which are clearly distinct, separated by insuperable intervals. This “increases the degree of rational perfection of which the final establishment of this hierarchy is capable.”[167] It is then under the influence of a purely formal motive that Comte’s preference is here decided. For he felt the strength and the import of Lamarck’s labours. Of the two celebrated antagonists, he said, Lamarck was unquestionably the one “who manifested the clearest and deepest sense of the true organic hierarchy.”[168]
Comte has even dealt with certain objections which do not go against Lamarck. Thus, we might think at first that, in his hypothesis, there is no real zoological series, since animal organisms would be essentially identical, their differences being henceforth attributed to the diverse and unequally prolonged influence of the external conditions. But, on looking into it more closely, we see, on the contrary, that this hypothesis only presents the series in a new aspect which would even render its existence still more evident. For the whole of the zoological series would then become, in fact as well as ideally, altogether analogous to the whole of the individual development, confined at least to its ascending period. It would then be conceived as continuous. “The progressive advance of the animal organism, which for us is only a convenient abstraction, would be converted into a natural law.”[169]