"I have observed, however, that, when circumstances demand, nature passes from one system to another without making a leap, provided they are allies. It is, indeed, by this faculty that she has come to form them all in succession, in proceeding from the simple to the more complex. "It is so true that she has the power, that she passes from one system to the other, not only in two different families which are allied, but she also passes from one system to the other in the same individual. "The systems of organization which admit as organs of respiration true lungs are nearer to systems which admit gills than those which require tracheæ. Thus not only does nature pass from gills to lungs in allied classes and families, as seen in fishes and reptiles, but in the latter she passes even during the life of the same individual, which successively possesses each system. We know that the frog in the tadpole state respires by gills, while in the more perfect state of frog it respires by lungs. We never see that nature passes from a system with tracheæ to a system with lungs. "_It is not the organs, i.e., the nature and form of the parts of the body of an animal, which give rise to the special habits and faculties, but, on the contrary, its habits, its mode of life, and the circumstances in which individuals are placed, which have, with time, brought about the form of its body, the number and condition of its organs, finally the faculties which it possesses._ * * * * * "Time and favorable circumstances are the two principal means which nature employs to give existence to all her productions. We know that time has for her no limit, and that consequently she has it always at her disposition. "As to the circumstances of which she has need (_besoin_) and which she employs every day to bring about variations in all that she continues to produce, we can say that they are in her in some degree inexhaustible. "The principal ones arise from the influence of climate, from that of different temperatures, of the atmosphere, and from all environing surroundings (_milieux_); from that of the diversity of places and their situations; from that of the most ordinary habitual movements, of actions the most frequent; finally from that of the means of preservation, of the mode of life, of defence, of reproduction, etc. "Moreover, as the result of these different influences the faculties increase and strengthen themselves by use, diversify themselves by the new habits preserved through long periods, and insensibly the conformation, the consistence--in a word, the nature and state of the parts and also of the organs--consequently participate in all these influences, are preserved and propagate themselves by generation" (_Système des Animaux sans Vertèbres_, p. 12). * * * * *