This is always a difficult undertaking, whether we start with the guess that B is caused by A or that it is not caused by A. In the case of water-finding, water is found at depths of 30 feet to 100 feet and more below the surface by engineers without the aid of "dousers" every day, and this is so frequent and regular a proceeding that the percentage of cases in which dousers find water, that is to say in which B--the discovery of water--follows A (A being the employment of a supposed sensitive douser with or without his twig) does not--so far as I am able to judge without strict statistical evidence--exceed the percentage of successes in searching and digging for water by ordinary intelligent men without the introduction of A. CHAPTER XL BIRTH-MARKS AND TELEGONY TWO widely-spread "beliefs"--in regard to the complicated and not generally familiar subject of the reproduction of animals--are, in addition to that dealt with in the last chapter, examples of the unjustified and primitive mode of forming a conclusion known as "post hoc ergo propter hoc." I refer, firstly, to the belief (which I have already mentioned) in the causation of what are called "birth-marks" by "maternal impressions," by which is meant the seeing of unusual and impressive things by the mother when with child; and, secondly, to the belief that a thoroughbred mare can be so affected or infected by the sire (say a zebra) of one foal as to convey to the foal of a later sire (say, a thoroughbred like herself) marks (such as stripes on the legs) which were not present in the second sire, though present in the first sire. This supposed occurrence is called "telegony," and is by some persons supposed to occur in dogs, cattle, and other animals, including man, as well as in the horse.