inheritance of a particular modification, and the possible inheritance of indirect results of that modification, or of changes correlated with it. This is a nice but crucial point on which most popular writers are confused. Let us examine it through a hypothetical case. A woman, not herself strong, bears a child that is weak. The woman then goes in for athletics, in order better to fit herself for motherhood; she specializes on tennis. After a few years she bears another child, which is much stronger and better developed than the first. "Look," some one will say, "how the mother has transmitted her acquirement to her offspring." We grant that her improved general health will probably result in a child that is better nourished than the first; but that is a very different thing from heredity. If, however, the mother had played tennis until her right arm was over-developed, and her spine bent; if these characteristics were nowhere present in the ancestry and not seen in the first child; but if the second child were born with a bent spine and a right arm of exaggerated musculature, we would be willing to consider the case on the basis of the inheritance of an acquired character. We are not likely to have such a case presented to us.
To put the matter more generally, it is not enough to show that some modification in the parent results in some modification in the child. For the purposes of this argument there must be a similar modification.
(8) Finally, data are frequently presented, which cover only two generations—parent and child. Indeed, almost all the data alleged to show the inheritance of acquired characteristics are of this kind. They are of little or no value as evidence. Cases covering a number of generations, where a cumulative change was visible, would be of weight, but on the rare occasions when they are forthcoming, they can be explained in some