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.
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.
There is little support in ordinary experience for the belief that birth-marks are caused by maternal impressions, although some of those who are concerned in a professional way with breeding operations cling to it. In very ancient times we find that there was a belief in it, as shown by the story of the patriarch Jacob, who, wishing to obtain the birth of spotted or parti-coloured lambs from a herd of sheep, placed in front of the breeding ewes stakes or rods from which he had removed the bark in rings, so as to make them parti-coloured. He was supposed to have been successful in this way in impressing the visual sense of the maternal ewes with "parti-colouration," and the belief was that they in consequence produced dappled or parti-coloured lambs. The belief, though not general, is widespread among simple folk that such influences can and do act on animals, and it has been, and is by some, similarly held that a human mother may be influenced by surrounding objects, so that if her surroundings are beautiful she will produce a beautiful child. There is absolutely no ground for this belief--based upon experiment. It is merely an unreasoning assumption of "after this, therefore because of this," based upon the incomplete observation of a few accidental cases of vague coincidence and a tenacious clinging to the belief that it is so because it is difficult to prove that it is not so. No trustworthy investigation or experiment on the subject is on record.