IT would appear that Prof. Lankester has not thought it worth while to read all the letters that have appeared in NATURE on the question raised by Sir Edward Fry, unless it is to be inferred from his remarks that he confines himself to the consideration of the arguments of those who have a place on the scientific Olympus of the Royal Society. In my letter, published December 6, I defended Lamarck's laws against the accusation that they were reciprocally destructive. Prof. Lankester reiterates his accusation without any further support. But this is not the whole question. In his last letter he suggests that acquired characters corresponding to Mr. Galton's definition should be taken, and an investigation made as to whether they are inherited or not in later generations. But in his former letter (November 29) he suggested very distinctly and deliberately that such an investigation was unnecessary, because the question was already settled. He has already condemned the heretic, and now consents to his trial. His words were—“Since the old character had not become fixed and congenital after many thousands of successive generations of individuals had developed it in response to environment, but gave place to a new character when new conditions operated on an individual, why should we suppose that the new character is likely to become fixed after a much shorter time of responsive existence?” To apply this once more to the case of pigment in relation to light. For thousands of generations no pigment has been developed, say, on the lower side of a flat fish, no light having fallen on it. The skin is experimentally exposed to light, and pigment appears: therefore the acquired character of absence of pigment, after thousands of generations, has produced no hereditary change, has not altered the potentialities of the tissue. The argument is fallacious, because the question of how much pigment is entirely ignored, and also the question how long the development of pigment experimentally takes. The force of the argument is entirely on the other side. Assume in this case, as Prof. Lankester does in his general argument, that the old character, the absence of pigment, is an acquired character. Then experiment has shown that this character is inherited: that is to say, the action of light obviously overcomes a resistance in producing pigment, and after years does not produce as much as on the upper side is present from the beginning. This resistance can be nothing else than heredity, the inheritance of a tendency to pigmentlessness. Therefore the acquired character is inherited. It is undeniable and indisputable that the argument propounded by Prof. Lankester proves the inheritance of acquired characters, if it is properly applied in accordance with the facts. This is on the assumption that the “old characters” are acquired. If they are not acquired, the argument has no force at all. The facts allow us to say that the tendency to pigmentlessness, or the resistance to the development of pigment on the lower side of a flounder, is certainly inherited, but whether or not it is due to the absence of light during many successive generations we do not know. As Sir Edward Fry says, if we by definition confine the term “acquired character” within the limits of an individual history, then of course an acquired character can never be inherited. The question is whether the conditions which produce a change in the individual can affect the offspring? The experimental investigation must take the following course. Suppose a given amount of stimulation X to act upon individuals in successive generations, producing in the first generation a result x. Then the question is if X remains the same, does x remain constant or not? If there is no inherited effect, then x must remain constant in all succeeding generations. If x increases by some amount, however small, and becomes x + a, then a is not acquired by the individual, but inherited, and it is clear that the result will go on increasing to x + 2a, x + 3a, and so on to x + na, where n represents the number of generations. In my own opinion, there is evidence that something of this kind does occur, though definite investigations are much to be desired.