"At present we clearly see that in order to bring about the existence of animals of all the classes, of all the orders, and of all the genera, nature has had to begin by giving existence to those which are the most simple in organization and lacking most in organs and faculties, the frailest in constituency, the most ephemeral, the quickest and easiest to multiply; and we shall find in the _amorphous_ or _microscopic polyps_ the most striking examples of this simplification of organization, and the indication that it is solely among them that occur the astonishing germs of animality. "At present we only know the principal law of the organization, the power of the exercise of the functions of life, the influence of the movement of fluids in the supple parts of organic bodies, and the power which the regenerations have of conserving the progress acquired in the composition of organs. "At present, finally, relying on numerous observations, seeing that with the aid of much time, of changes in local circumstances, in climates, and consequently in the habits of animals, the progression in the complication of their organization and in the diversity of their parts has gradually operated (_a dû s'opérer_) in a way that all the animals now known have been successively formed such as we now see them, it becomes possible to find the solution of the following question: "What is a _species_ among living beings? "All those who have much to do with the study of natural history know that naturalists at the present day are extremely embarrassed in defining what they mean by the word species. "In truth, observation for a long time has shown us, and shows us still in a great number of cases, collections of individuals which resemble each other so much in their organization and by the _ensemble_ of their parts that we do not hesitate to regard these collections of similar individuals as constituting so many species. "From this consideration we call _species_ every collection of individuals which are alike or almost so, and we remark that the regeneration of these individuals conserves the species and propagates it in continuing successively to reproduce similar individuals. "Formerly it was supposed that each species was immutable, as old as nature, and that she had caused its special creation by the Supreme Author of all which exists. "But we can impose on him laws in the execution of his will, and determine the mode which he has been pleased to follow in this respect, so it is only in this way that he permits us to recognize it by the aid of observation. Has not his infinite power created an order of things which successively gives existence to all that we see as well as to all that which exists and which we do not know?