CHAPTER XIII

in reporting cases which were assumed to demonstrate heredity. The child was so and so; it was found on inquiry that the father was also so and so: Post hoc, ergo propter hoc—it was heredity. Such a method of investigation is calculated to bring genetics into disrepute, and would hazard the credit of genealogy. As a fact, one case counts for practically nothing as proof of hereditary influence; even half a dozen or a dozen may be of no significance. There are two ways in which genealogical data can be analyzed to deduce biological laws: one is based on the application of statistical and graphic methods to the data, and needs some hundreds of cases to be of value; the other is by pedigree-study, and needs at least three generations of pedigree, usually covering numerous collaterals, to offer important results. It is not to be supposed that anyone with a