Constant, or free, life is the third form of life; it belongs to the most highly organized animals. In it, life is not suspended in any circumstance, it unrolls along a constant course, apparently indifferent to the variations in the cosmic environment, or to the changes in the material conditions that surround the animal.

Leçons sur les Phénomènes de la Vie Communs aux Animaux et aux Végétaux (1878-1879)