The want of resemblance in descent observed in pathological and teratological families evidences the want of embryogenic energy which is so accentuated in those families as to end in sterility after a few generations. The attenuation of embryogenic force which may be signalized by failures of very different elements may serve to interpret what is called dissimilar morbid heredity and that paradoxical heredity designated as collateral morbid heredity.
It should be remarked that dissimilarity in morbid families is not absolutely fortuitous. The head of a family gives rise to offspring suffering from different and differently seated disorders of evolution, that cause various morbid predispositions, the variety of which is, however, not so great but that we can find analogies in the manifestation capable of giving a family resemblance to them. Degeneration, in fact, does not take effect except under a kind of rule. As Morel has well observed, unlike degenerates of one family resemble unlike degenerates of another family to such an extent that, like monsters, they are susceptible, wheresoever they may come from, of a scientific classification. Degeneration has its laws the same as normal evolution; whatever may be its cause, it is manifested under a relatively restricted number of common forms.
The theory of the teratological origin of manifestations of morbid heredity is really the only one that will allow us to explain how very diverse conditions of generation, such as extreme youth or too advanced age of progenitors, disproportion in their ages, permanent or even transitory disorders in their vitality, drunkenness, intoxications, infections, accidental exhaustion of the nervous system, or acquired neurasthenia, can produce the same effects as morbid heredity. We should not, in fact, be surprised at finding that degenerates by heredity are not different from degenerates in consequence of disorders of nutrition in progenitors, since degeneration in general results from embryogenical troubles which are reduced, as a whole, to troubles of nutrition. The teratological theory of morbid heredity and of degeneration permits us to comprehend not only unlikeness in morbid heredity, but also the absence of heredity in diseases of the group presumed to be hereditary, but which might be more correctly called degenerative.
Greater importance attaches to disorders of development, when we regard their consequences, as they are produced at a period nearer the beginning of the evolution. External forms are fixed long before the structure of the organs has reached perfection. Thus in man birth finds some parts of the nervous system and the most important ones, when the light of relation is regarded in full development. It is therefore easily compre-
