Darwin attached great importance to the existence in man of so-called rudimentary organs, which he regarded as convincing evidence of man’s descent from the lower forms of animal life. Nineteenth century science, being ignorant of the functional purpose served by many organs, arbitrarily pronounced them to be useless organs, and chose, in consequence, to regard them all as the atrophied and (wholly or partially) functionless remnants of organs that were formerly developed and fully functional in remote ancestors of the race. Darwin borrowed this argument from Lamarck. It may be stated thus: Undeveloped and functionless organs are atrophied organs. But atrophy is the result of disuse. Now disuse presupposes former use. Consequently, rudimentary organs were at one time developed and functioning, viz. in the remote ancestors of the race. Since, therefore, these selfsame organs are developed and functional in the lower forms of life, it follows that the higher forms, in which these organs are reduced and functionless, are descended from forms similar to those in which said organs are developed and fully functional. This argument, however, fairly bristles with assumptions that are not only wholly unwarranted, but utterly at variance with actual facts. In the first place, it wrongly assumes that all reduced organs are functionless, and, conversely, that all functionless organs are atrophied or reduced. Facts, however, prove the contrary; for we find frequent instances of reduced organs which function, and, _vice versa_, of well-developed organs which are functionless. The tail, for example, in cats, dogs, and certain Catarrhine monkeys, though it discharges neither the prehensile function that makes it useful in the Platyrrhine monkey, nor the protective function that makes it useful to horses and cattle in warding off flies, is, nevertheless, despite its inutility or absence of function, a quite fully developed organ. Conversely, the reduced or undeveloped fin-like wings of the penguin are by no means functionless, since they enable this bird to swim through the water with great facility.