130 THE DESCENT OF MAN. Part I. 



to those of rudimentary organs given in the first chapter, 

 that many of them might have been indifferently intro- 

 duced in either chapter. Thus a human uterus furnished 

 with cornua may be said to represent in a rudimentary 

 condition the same organ in its normal state in certain 

 mammals. Some parts which are rudimental in man, 

 as the os coccyx in both sexes and the ruanimse in the 

 male sex, are always present ; whilst others, such as 

 the supracondyloid foramen, only occasionally appear, 

 and therefore might have been introduced under the 

 head of reversion. These several reversionary, as well 

 as the strictly rudimentary, structures reveal the de- 

 scent of man from some lower form in an unmistakeable 

 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 governed by some earlier developed part. Various 

 monstrosities, as I. Geoffroy repeatedly insists, are thus 

 intimately connected. Homologous structures are par- 

 ticularly 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 colour of the skin and 

 hair, colour and constitution, are more or less correlated. 49 

 Professor Schaaffhausen first drew attention to the rela- 



49 The authorities for these several statements are given in my 

 Variation of Animals under Domestication,' vol. ii. p. 320-335. 



