EMBRYONIC IDENTITY OF VERTEBRATES 167 



present, therefore — because they are acquired before birth — the appearance 

 of being hereditary. Congenitally acquired characters, in so far as they 

 affect the body only, cannot be hereditarily transmitted, any more than 

 any other somatic characters. ^' 



If teratological characters are not hereditary, but only congenital, their 

 explanation is simple enough — theoretically, at least. Von Baer showed 

 that the embryos of aU the vertebrates exhibit, at their origin, one common 

 tjrpe ; and that they go through a series of stages, identical in every species, 

 before eventually differentiating themselves. Now, if, as is very widely 

 held, teratological characters represent a reversion from the human type 

 to some inferior animal type, the identity of form observable in the embryo 

 of aU the vertebrates in their earliest stages renders these characters com- 

 prehensible. As Fere remarks (pp. 212, 213), this common origin makes 

 it probable that the embryo of every vertebrate contains in potentia the 

 organs of all the others ; and that, if the development of the embryo be 

 accidentally disturbed by any cause, an organ belonging to one species may 

 develop in the embryo of another species, or an organ which is constant in 

 the species may disappear in an individual. " Quelques anomahes," con- 

 tinues Fere, " que nous retrouvons chez les types degeneres de I'humanit^, 

 rapellent des formes appartenant a des etres moins eleves que I'homme et 

 meme tres eloignes de lui. Mais un trouble de I'embryogenfese peut 

 produire les memes anomahes de 1' organisation, la teratogenie experi- 

 mentale le montre surabondamment." 



The question remains whether the teratological malformations are the 

 cause which predisposes to the organic or functional disease ; or whether 

 they are themselves the morphological symptoms of an underlying con- 

 stitutional degeneracy, which manifests itself at the same time in the 

 disease which generally accompanies the malformation. 



In the first place, it is certain that there is no causal relation implying 

 a necessary correspondence between a given malformation and a given 

 disease. It seems as if the same malformations may accompany several 

 different diseases, and vice versa. In the second place. Fere accepts the 

 view of John Hunter, that there is, in reality, no such thing as inheritance 

 of diseases itself, but only inheritance of the morbid predisposition of 

 which disease is the outcome. Fere admits that this view is correct, that 

 '' the facts, as a general rule, support it." If Hxmter's theory be correct, 

 it would seem as if — originally, at all events — it is the morbid constitution 

 of the parent that is transmitted, and that this brings about a tera- 

 tological manifestation in the offspring. Teratology would thus be the 

 expression of a fundamental pathological disposition ; the latter is the 

 conditio sine qua non of the former. 



Developing this idea, which is undoubtedly correct — that there is no 

 teratological manifestation without an underlying morbid predisposition — 

 it might be urged that what is hereditary is not the teratological mal- 



