The influence exerted by Jacob's rods was of a different character from that which is supposed to give rise to the marking of offspring before birth, which is not uncommon if we are to accept as true all the cases mentioned in books referring to the subject. What occurred took place _before_ conception, and not subsequent to it, as in these cases. Nevertheless, both classes of phenomena are recognized by so competent an authority as M. Th. Ribot, who, in his "Heredity,"[57:A] when criticising Dr. Lucas' explanation of the origin of the numerous exceptions to the law of heredity, as being due to the operation of the law of spontaneity, affirms that there is no law of spontaneity, but that all such exceptions may be explained by reference to certain causes of diversity. M. Ribot gives three causes of diversity, which are: 1--Antagonistic heredities of two parents; 2--Accidental causes in action at the moment of generation; 3--External and internal influences subsequent to conception. He assigns but little importance to causes acting after birth, such as diet, climate, circumstances, education, physical and moral influences, because, though they may produce serious effects, these are not radical. Possibly, however, since the advance made in the education of those who are born with defects of the sensory apparatus, M. Ribot would somewhat modify his opinion on that point. As to the causes which operate at the period of conception, or subsequent thereto and before birth, he says, in relation to the latter class, they "are all the physical and moral disturbances of uterine existence--all those influences which can act through the mother upon the fetus during the period of gestation; impressions, emotions, defective nutrition, effects of imagination." He adds: "These causes are very real, despite the objections of Lucas, who attacks them in order to establish his law of spontaneity. We see from examples that between considerable causes and their effects there exists an amazing disproportion." The causes of diversity which operate at the instant of conception depend, says Ribot, "less upon the physical and moral natures of the parents than on the particular state in which they are at the moment of procreation." This fact is referred to by M. de Quatrefages as fully proving the universality of the law of heredity, and M. Ribot adds, "It enables us to understand that those transitory states which exist at the moment of conception may exert a decisive influence on the nature of the being procreated, so that often, where now we see only spontaneity, a more perfect knowledge of the causes at work would show us heredity."