Chap. XIII. 



REVERSION. 



57 



Monstrosities. — A large number of monstrous growths and of 

 lesser anomalies are admitted by every one to be due to an 

 arrest of development, that is, to the persistence of an em- 

 bryonic condition. If every horse or ass had striped legs whilst 

 young, the stripes which occasionally appear on these animals 

 when adult would have to be considered as due to the ano- 

 malous retention of an early ' character, and not as due to 

 reversion. Now, the leg-stripes in the horse-genus, and some 

 other characters in analogous cases, are apt to occur during 

 early youth and then to disappear ; thus the persistence of early 

 characters and reversion are brought into close connection. 



But many monstrosities can hardly be considered as the result 

 of an arrest of development ; for parts of which no trace can be 

 detected in the embryo, but which occur in other members of 

 the same class of animals or plants, occasionally appear, and 

 these may probably with truth be attributed to reversion. For 

 instance : supernumerary mammas, capable of secreting milk, 

 are not extremely rare in women; and as many as five have 

 been observed. When four are developed, they are generally 

 arranged symmetrically on each side of the chest ; and in one 

 instance a woman (the daughter of another with supernumerary 

 mammae) had one mamma, which yielded milk, developed in 

 the inguinal region. This latter case, when we remember the 

 position of the mammas in some of the lower animals on both 

 the chest and inguinal region, is highly remarkable, and leads 

 to the belief that in all cases the additional mammae in woman 

 are due to -reversion. The facts given in the last chapter on 

 the tendency in supernumerary digits to regrowth after amputa- 

 tion, indicate their relation to the digits of the lower vertebrate 

 animals, and lead to the suspicion that their appearance may in 

 some manner be connected with reversion. But I shall have to 

 recur, in the chapter on pangenesis, to the abnormal multipli- 

 cation of organs, and likewise to their occasional transposition. 

 The occasional development in man of the coccygeal vertebrae 

 into a short and free tail, though it thus becomes in one sense 

 more perfectly developed, may at the same time be considered 

 as an arrest of development, and as a case of reversion. The 

 greater frequency of a monstrous kind of proboscis in the pig 

 than in any other mammal, considering the position of the pig 



