MANNER OF DEVELOPMENT. 35 



We are thus reminded of the delight shown by almost all boys 

 in climbing trees; and this again reminds us how lambs and kids, 

 originally alpine animals, delight to frisk on any hillock, however 

 small. Idiots also resemble the lower animals in some other 

 respects; thus several cases are recorded of their carefully smell- 

 ing every mouthful of food before eating it. One idiot is described 

 as often using his mouth in aid of his hands, whilst hunting for 

 lice. They are often filthy in their habits, and have no sense of 

 decency; 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 structure in some lower and 

 adult member of the same group, it may in one sense be con- 

 sidered as a case of reversion. The lower members in a group 

 give us some idea how the common progenitor was probably con- 

 structed; and it is hardly credible that a complex part, arrested 

 at an early phase of embryonic development, should go on grow- 

 ing so as ultimately to perform its proper function, unless it had 

 acquired 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 rever- 

 sion.^' There are other cases which come more strictly under our 



37 Prof. Laycock sums up the character of brute-like idiots by calling- 

 them "theroid;" 'Journal of Mental Science,' July, 1863. Dr. Scott ('The 

 Deaf and Dumb,' 2nd edit., 1870, p. 10) has often observed 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. 



^ In my 'Variation of Animals under Domestication' (vol. 11. 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 mammae being generally placed symmetrically on the 

 breast; and more especially from one case, in which a single efficient 

 mamma occurred in the inguinal region of a woman, the daughter of 

 another woman with supernumerary mammae. But I now find (see, 

 for instance. Prof. Preyer, 'Der Kampf um das Dasein,' 1859, s. 45) that 

 mammae erraticae 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 child was thus nourished. The probability 

 that the additional mammae are due to reversion is thus much weak- 

 ened; nevertheless it still seems to me probable, because tv,'o pairs 

 are often found symmetrically on the breast; and of this I myself 

 have received information in several cases. It is well known that some 

 Lemurs normally have two pairs of mammae on the breast. Five 

 cases have been recorded of the presence of more than a pair of mam- 



