130 
THE DESCENT OF M AN . 
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 mammae 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 . 48 
Professor Sehaaffhausen first drew attention to the rela- 
45 The authorities for these several statements are given in my 
Variation of Animals under Domestication,’ vol. ii. p. 320-335. 
