In this contrast, on the whole justified, between the importance of variations in the "vegetative" and variations in the "animal" parts, one may see without doing violence to Buffon's thought, an indication of the difference between homology and analogy. It is usually in the external parts, in the organs by which the animal adapts itself to its environment, that one meets with the greatest number of analogical resemblances. This contrast of vegetative and animal parts and their relative importance for the discovery of affinities was at any rate a considerable step towards an analysis of the concept of unity of plan. To Xavier Bichat (1771-1802) belongs the credit of working out in detail the distinction drawn by Aristotle and Buffon between the animal and the vegetative functions. Bichat was not a comparative anatomist; his interest lay in human anatomy, normal and pathological. So his views are drawn chiefly from the consideration of human structure. He classifies functions into those relating to the individual and those relating to the species. The functions pertaining to the individual may be divided into those of the animal and those of the organic life.[37] "I call _animal life_ that order of functions which connects us with surrounding bodies; signifying thereby that this order belongs only to animals" (p. lxxviii.). Its organs are the afferent and efferent nerves, the brain, the sense-organs and the voluntary muscles; the brain is its central organ. "Digestion, circulation, respiration, exhalation, absorption, secretion, nutrition, calorification, or production of animal heat, compose organic life, whose principal and central organ is the heart" (p. lxxix.).