processes of growth have been determined. He never expects a muscular tissue to develop into a skeleton, a nervous tissue into a gland, an osseous tissue into a sensory organ. He never expects a tail to become a hand or a foot, though he sees it in monkeys and marsupials serving the offices of prehension and locomotion. He never expects to find fingers growing anywhere except from metacarpal bones, or an arm developed from a skull. The well-known generalization of Geoffroy St. Hilaire that an organ is more easily annihilated than transposed, points to the fundamental law of Epigenesis. In the same direction point all the facts of growth. Out of a formless germinal membrane we see an immense variety of forms evolved; and out of a common nutritive fluid this variety of organs is sustained, repaired, replaced; and this not indifferently, not casually, but according to rigorous laws of succession; that which precedes determining that which succeeds as inevitably as youth precedes maturity, and maturity decay. The nourishment of various organs from plasmodes derived from a common fluid, each selecting from that fluid only those molecules that are like its own, rejecting all the rest, is very similar to the formation of various crystals in a solution of different salts, each salt separating from the solution only those molecules that are like itself. Reil long ago called attention to this analogy. He observed that if in a solution of nitre and sulphate of soda a crystal of nitre be dropped, all the dissolved nitre crystallizes, the sulphate remaining in solution; whereas on reversing the experiment, a crystal of sulphate of soda is found to crystallize all the dissolved sulphate, leaving the nitre undisturbed. In like manner muscle selects from the blood its own materials which are there in solution, rejecting those which the nerve will select.