The reproach that I preach blind faith in the progress of science, destined to replace religious faith, is unjust, because my faith depends on a confidence that science has already deserved. Equally unjust is the reproach that I have built my system on a partly metaphysical principle. According to M. Parodi,[232] the hypothesis of physiological old age and of natural death seem to “involve the idea of a natural duration of human life, which, however, from accidental reasons man does not complete at present. M. Metchnikoff repeatedly uses the expression ‘normal cycle.’ Now do we not see here the surreptitious repetition of the old teleological conception of nature, although at first he so energetically disavowed it? It is the belief that the species is a necessary reality, corresponding to a definite type of its own, in fact a special design of nature; that nature, to guide herself, had an ideal which circumstances could mistake or degrade, but which had to be restored to its perfect form? Otherwise, why does he insist that there must be a condition of perfect and stable equilibrium between individual and environment? that there is a normal cycle and that it must be possible to harmonise the disharmonies?” I can show easily that all these objections rest upon a simple misunderstanding. I have never conceived of the existence of any ideal of nature or of the inevitable necessity of transforming disharmonies to harmonies. I have no knowledge of the “designs” and “motives” of nature; I have never taken my stand on metaphysical ground. I have not the remotest idea if nature has any ideal and if the appearance of man on the earth were a part of such an ideal. What I have spoken of is the ideal of man corresponding to the need to ward off the great evils of old age as it is now, and of death as we see it around us. I have said, moreover, that human nature, that collection of complex features of multiple origin, contains certain elements which may be used to modify it according to our human ideal. I have done nothing but what the horticulturist does when he finds in the nature of plants elements which suggest to him to try and make new and improved races. Just as the constitution of some plum trees contains elements which make it possible to produce plums without stones which are pleasanter to eat, so also in our own nature there exist characters which make it possible to transform our disharmonious nature into a harmonious one, in accordance with our ideal, and able to bring us happiness. I have not the smallest idea what ideal nature may have on the subject of plums, but I know very well that man has such designs and such an ideal as form a point of departure for the transformation of the nature of plums. Substitute man for the plum tree and you are at my point of view. When I have spoken of the normal cycle of life or of physiological old age, I have used the words normal and physiological only in relation to our ideal of the human constitution.