_Proof of the Monistic Theory._—On the other hand, under the disguise of living forms, the physiologist recognizes the existence of an identical basis. His trained ear marks amid the overcharged instrumentation of the vital work the recognizable undertones of a constant theme. It was the work of Claude Bernard to bring this common basis to light. He shows that plants live as animals do, that they breathe, digest, have sensory reactions, move essentially like animals, destroy and build up in the same manner the immediate chemical principles. For that purpose it was necessary to pass in review, examining them from their foundation and distinguishing the essential from the secondary, the different vital manifestations—digestion, respiration, sensibility, motility, and nutrition. This is what Claude Bernard did in his work _Sur les Phénomènes de la vie communs aux animaux et aux plantes_. We need only to sketch in broad outline the characteristic features of his lengthy demonstration. _Unity in the Formation of Immediate Chemical Principles._—The first and most important of the differences pointed out between the life of animals and that of plants was relative to the formation of immediate principles. On this ground, indeed, vital dualism raised its fortress. The animal kingdom was considered in its totality as the parasite of the vegetable kingdom. To J. B. Dumas, animals, whatever they may be, make neither fat nor any elementary organic matter; they borrow all their foods, whether they be sugars or starches, fats or nitrogenous substances, from the vegetable kingdom. About the year 1843 the researches of the chemists, and of Payen in particular, succeeded in proving the presence, almost constant, of fatty matters in vegetables; and, further, these matters existed there in proportions more than sufficient to explain how the beast which fed upon them was fattened. The chemists attributed to nature as much practical sense as they themselves possessed; and since the hay and the grass of the ration brought fat ready made to the horse, the cow, and the sheep, they declared that the animal organism had nothing whatever to do but to put this food into the tissues, or to arrange for it to pass into the milk. But nature is not so wise and economical as was supposed at the Académie des Sciences. After a memorable debate, in which Dumas, Boussingault, Payen, Liebig, Persoz, Chossat, Milne-Edwards, and Flourens took part, and, later on, Berthelot and Claude Bernard, it was agreed that the animal does not grow fat from the fatty food which is supplied it, and that it makes its own fat just as the vegetable does, but in another manner. In the same way sugar, the normal constituent substance necessary for the nutrition of animals and plants, instead of being a vegetable product passing by alimentation from the herbivorous animals and thence to the carnivorous, is manufactured by the animal itself. Generally speaking, immediate principles have an equal claim