THE DESCENT OR ORIGIN OF MAN 67 



many of them might have been indifferently introduced 

 either there or here. Thus a human uterus furnished with 

 cornua may be said to represent, in a rudimentary condi- 

 tion, 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 sjipra- 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 un- 

 mistakable manner. 



Correlated Variation. — In man, as in the lower animals, 

 many structureis 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. Geof- 

 frey 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 re- 

 marked, 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 leg. 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 cor- 

 related." Prof. Schaaffhausen first drew attention to the 

 relation apparently existing between a muscular 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 spon- 



«5 The authorities for these several statements are given in my "Variation 

 of Animals under Domestication," vol. ii. pp. 320-33.5. 



