MANNER OP DEVELOPMENT. 41 



animals in some other respects; thus several cases are re- 

 corded of their carefully smelling every mouthful of food 

 before eating it. One idiot is described as often using his 

 mouth in aid of his hands while hunting for lice. They 

 are often filthy in their habits, and have no sense of de- 

 cency; and several cases have been published of their bodies 

 being remarkably hairy.* 



Reversion. Many of the cases to be here given, might 

 have been introduced under the last heading. When a 

 structure is arrested in its development, but still continues 

 growing, until it closely resembles a corresponding struct- 

 ure in some lower and adult member of the same group, it 

 may in one sense be considered as a case of reversion. The 

 lower members in a group give us some idea how the com- 

 mon progenitor was probably constructed; and it is hardly 

 credible that a complex part, arrested at an early phase of 

 embryonic development, should go on growing so as ulti- 

 mately to perform its proper function, unless it had ac- 

 quired such power during some earlier state of existence, 

 when the present exceptional or arrested structure was 

 normal. The simple brain of a microcephalous idiot, in 

 as far as it resembles that of an ape, may in this sense be 

 said to offer a case of reversion.! There are other cases 



* Prof. Lavcock sums up the character of brute-like idiots by call- 

 ing them theroid ; *' Journal of Mental Science," July, 1863. Dr. 

 Scott ("The Deaf and Dumb," 2d edit., 1870, p. 10) has often ob- 

 served the imbecile smelling their food. See, on this same subject, 

 and on the hairiness of idiots. Dr. Maudsley, "Body and Mind," 

 1870, pp. 46-51. Pinel has also given a striking case of hairiness in 

 an idiot. 



f In my "Variation of Animals under Domestication" (vol. ii, p. 

 57), I attributed the not very rare cases of supernumerary mammae 

 in women to reversion. I was led to this as a probable conclusion, 

 by the additional maramse being generally placed symmetrically on 

 the breast; and more especially from one case, in which a single effi- 

 cient mammae occurred in the inguinal region of a woman, the daugh- 

 ter of another woman with supernumerary mammae. But I now find 

 (see, for instance, Prof. Preyer, ' Der Kampf urn das Dasein," 1869, 

 s. 45) that mamnup errnticm occur in other situations, as on the back, 

 in the armpit, and on the thigh; the mammae in this latter instance 

 having given so much milk that the <;hild was thus nourished. The 

 probability that the additional mammae are due to reversion is thus 

 much weakened ; nevertheless, it still seems to me probable, because 

 two i)airs are <>lten found symmetrically on the breast ; and of this I 

 myself have received information in several cases. It is well known 



