MANNER OF DEVELOPMENT. 49 



may be said to represent, in a rudimentary condition, the 

 same organ in its normal state in certain mammals. Some 

 parts which are rudimentary in man, as the os coccyx in 

 both sexes, and the mammae in the male sex, are always 

 present; while others, such as the supra-condyloid foramen, 

 only occasionally appear, and therefore might have been in- 

 troduced under the head of reversion. These several rever- 

 sionary structures, as well as the strictly rudimentary ones, 

 reveal the descent of man from some lower form in an 

 unmistakable manner. 



Correlated Variation. In man, as in the lower ani- 

 mals, many structures are so intimately related, that when 

 one part varies so does another, without our being able, in 

 most cases, to assign any reason. We cannot say whether 

 the one part governs the other, or whether both are gov- 

 erned by some earlier developed part. Various monstrosi- 

 ties, as I. Geoffrey repeatedly insists, are thus intimately 

 connected. Homologous structures are particularly liable to 

 change together, as we see on the opposite sides of the body, 

 and in the upper and lower extremities. Meckel long ago 

 remarked, that when the muscles of the arm depart from 

 their proper type, they almost always imitate those of the 

 leg; and so, conversely, with the muscles of the legs. The 

 organs of sight and hearing, the teeth and hair, the color 

 of the skin and of the hair, color and constitution, are 

 more or less correlated.* Prof. Schaaffhausen first drew 

 attention to the relation apparently existing between a mus- 

 cular frame and the strongly pronounced supra-orbital 

 ridges, which are so characteristic of the lower races of man. 



Besides the variations which can be grouped with more 

 or less probability under the foregoing heads, there is a 

 large class of variations which may be provisionally called 

 spontaneous, for to our ignorance, they appear to arise 

 without any exciting cause. It can, however, be shown 

 that such variations, whether consisting of slight individ- 

 ual differences, or of strongly marked and abrupt devi- 

 ations of structure, depend much more on the constitu- 

 tion of the organism than on the nature of the conditions 

 to which it has been subjected, f 



*The authorities for these several statements are given in my 

 "Variation of Animals under Domestication," voL ii, pp. 320-335. 



f This whole subject has been discussed in chap, xxiii, vol. ii, of 

 my " Variation of Animals and Plants under Domestication." 



