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] For the logical perfection of science, Comte prefers to regard species as fixed in the absence of contrary proofs. None the less Lamarck has stated a problem of the highest interest. Comte points out its importance. “The rational theory of the necessary action of the various _milieux_ on the different organisms has still almost entirely to be formulated. Such an order of research, although greatly neglected, constitutes one of the finest subjects which the present condition of biology can present.” By this means, he adds, we might obtain a theory for the perfecting of living species even including mankind.[170] V.