It does not seem seriously wrong to suppose that there are some individuals at any age who have no more mental ability than the baby of the poorest mental ability at birth. At any rate our intelligence scales are hardly fine enough to measure the difference in intellectual capacity between the dullest adult idiots and the dullest idiot babies. We shall, therefore, here assume that mental capacity extends to zero at each age. The importance of this will be evident when we consider the question whether the distributions of ability are symmetrical around the average point at each age. Postponing for the present the discussion of unsymmetrical or skewed distributions, we may consider the several meanings of stages of development. In applying the concept of the probability curve we should distinguish between individuals who have attained their mature mental capacity and those who are still maturing. The former would be represented by a random group of adults (Distribution E, Fig. 3) the latter by a group of nine-year-olds (Distribution C). If we say, for example, that a child has reached a certain stage of development we might have in mind the final distribution of mature capacity or the distribution of capacity among those of his particular age or of all ages. When we compare stages of development we must, therefore, be careful to indicate the distribution surface to which we are referring. An increase in development may refer to at least five different things depending upon the scale of measurement to which reference is made. Besides an increase measured by the physical scale, the scales for adults, for the immature or for all ages, to which we have already referred, it may mean an increase judged by the distribution of increases which individuals of the same life-age and capacity make in the same period of time. This last meaning may be the most significant, although it has never been used. It has reference to a distribution surface of _increases_ such as is represented in Distribution F, Fig. 3. This is intended to show the increases in one year of all two-year-old children who had average ability at 2 years, on the assumption that at 3 years these children would on the average equal the average of all three-year-olds. It is clear that when these increases are measured in objective units the latter have a still different significance from that assigned to them in connection with other scales. An increase of one objective unit here might represent twice the standard deviation, while it only represents 0.2 of the standard deviation in another distribution. (b) THE YEAR UNIT OF THE BINET SCALE. A sharp disagreement of opinion as to whether the Binet year units can be regarded equivalent has arisen between Karl Pearson, Director of the Galton Laboratory in London, and certain psychologists who have used the Binet scale. Cyril Burt, for example, says, as quoted by Pearson: