Lamarck's approach: (1) In every animal which has not arrived at maturity, the increased and continued employment of any organ strengthens that organ gradually, develops it, enlarges it, and gives it a power proportional to the duration of its employment: on the other hand, the continued disuse of any organ gradually weakens it, deteriorates it, progressively diminishes its faculties, and finally causes it to disappear. (2) Every feature which, under natural conditions, individuals have gained or lost by the action of circumstances to which their race has been for some time exposed: as, for instance, the results of excessive use or disuse of an organ - is preserved in reproduction and transmitted to the offspring provided that the acquired changes were present in both parents. The small changes thus produced and transmitted from generation to generation are increased in successive generations by the action of the same causes which originated them, and thus in long periods of time the form and structure of the descendants of an ancestral organism may be completely changed as compared with the form and structure of the ancestor. Given sufficient time, these small changes can have produced man and the higher animals from simple primitive protoplasmic animalcules. = = = = = Darwin's approach: (1) All plants and animals produce offspring which resemble their parents on the whole; these offspring, however, exhibit also new and individual features differing from those of their parents (congenital variations). (2) In Nature there is a severe struggle for existence. Only one pair out of the many thousands often produced by a pair of plants or animals survive to maturity, and in their turn produce offspring. (3) The survivors are those whose congenital variations enabled them to gain advantage over their fellows. (4) The surviving forms may be almost exactly like their parents, but often a departure from the parental form must be an advantage, however small. Such departure, or variation, when INBORN or CONGENITAL, not only enables its possessor to survive and produce offspring, but is handed on by heredity to that offspring. (5) A successful congenital variation is intensified in the new generation bred from parents in both of which it had congenitally appeared. (6) By this process of natural selection of advantageous congenital variations, operating in countless millions of successive generations, the transformation of simple into more elaborate forms of life has been effected. The real difference between Lamarck's and Darwin's theories was then explained. Congenital variation is an admitted and demonstrable fact; transmission of congenital variations is also an admitted and demonstrable fact. Change of structure acquired during life - as stated by Lamarck - is also a fact, though very limited. But the transmission of these latter changes to offspring is NOT PROVED EXPERIMENTALLY.= = = = = I like the summation of Lamarck as use it or lose it. Distinctly congruent with Natural Law. The last sentence is crucial. Now the transmission of those changes HAS been proved experimentally, and in many cases the exact mechanism of the transmission is known. Like Ptolemy vs Copernicus, the truth is that both approaches are right. Nature doesn't miss a trick.
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