"As to the circumstances of which she has had need and of which she makes use every day in order to cause her productions to vary, we can say that in a manner they are inexhaustible. "The essential ones arising from the influence and from all the environing media, from the diversity of local causes, of habits, of movements, of action, finally of means of living, of preserving their lives, of defending themselves, of multiplying themselves, etc. Moreover, as the result of these different influences, the faculties, developed and strengthened by use, become diversified by the new habits maintained for long ages, and by slow degrees the structure, the consistence--in a word, the nature, the condition of the parts and of the organs consequently participating in all these influences, became preserved and were propagated by heredity (génération)." (Packard's translation.) Salient Points.--The salient points in Lamarck's theory may be compacted into a single sentence: It is a theory of the evolution of animal life, depending upon variations brought about mainly through use and disuse of parts, and also by responses to external stimuli, and the direct inheritance of the same. His theory is comprehensive, so much so that he includes mankind in his general conclusions. Lamarck supposed that an animal having become adapted to its surroundings would remain relatively stable as to its structure. To the objection raised by Cuvier that animals from Egypt had not changed since the days when they were preserved as mummies, he replied that the climate of Egypt had remained constant for centuries, and therefore no change in its fauna was to be expected. Species.--Since the question of the fixity of species is the central one in theories of evolution, it will be worth while to quote Lamarck's definition of species: "All those who have had 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.... 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." He then goes on with a long discussion to show that large collections of animals exhibit a great variation in species, and that they have no absolute stability, but "enjoy only a relative stability."