"Indeed, if the manner (_usage_) of life tends to develop the organization, and even to form and multiply the organs, as the state of an animal which has just been born proves it, compared to that where it finds itself when it has reached the term where its organs (beginning to deteriorate) cease to make new developments; if, then, each particular organ undergoes remarkable changes, according as it is exercised and according to the manner of which I have shown you some examples, you will understand that in carrying you to the end of the animal chain where are found the most simple organizations, and that in considering among these organizations those whose simplicity is so great that they lie at the very door of the creative power of nature, then this same nature--that is to say, the state of things which exist--has been to form directly the first beginnings of organization; she has been able, consequently, by the manner of life and the aid of circumstances which favor its duration, to progressively render perfect its work, and to carry it to the point where we now see it. "Time is wanting to present to you the series of results of my researches on this interesting subject, and to develop-- "1. What really is life. "2. How nature herself creates the first traces of organization in appropriate groups where it had not existed. "3. How the organic or vital movement is excited by it and held together with the aid of a stimulating and active cause which she has at her disposal in abundance in certain climates and in certain seasons of the year. "4. Finally, how this organic movement, by the influence of its duration and by that of the multitude of circumstances which modify its effects, develops, arranges, and gradually complicates the organs of the living body which possesses them. "Such has been without doubt the will of the infinite wisdom which reigns throughout nature; and such is effectively the order of things clearly indicated by the observation of all the facts which relate to them." (End of the opening discourse.) APPENDIX (p. 141). _On Species in Living Bodies._ "I have for a long time thought that _species_ were constant in nature, and that they were constituted by the individuals which belong to each of them. "I am now convinced that I was in error in this respect, and that in reality only individuals exist in nature. "The origin of this error, which I have shared with many naturalists who still hold it, arises from _the long duration_, in relation to us, _of the same state of things_ in each place which each organism inhabits; but this duration of the same state of things for each place has its limits, and with much time it makes changes in each point of the surface of the globe, which produces changes in every kind of circumstances for the organisms which inhabit it.