42 ^HE DESCENT OF MAN. 



On the other hand, if man is descended from some ape-like crea- 

 ture, no valid reason can be assigned why certain muscles should 

 not suddenly reappear after an interval of many thousand genera- 

 tions, in the same manner as with horses, asses, and mules, dark- 

 colored stripes suddenly reappear on the legs and shoulders, after 

 an interval of hundreds, or more probably of thousands of genera- 

 tions. 



These various cases of reversion are so closely related to those 

 of rudimentary organs given in the first chapter, that 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 condition, the same organ in its 

 normal state in certain mammals. Some parts which are rudi- 

 mentary in man, as the os coccyx in both sexes, and the mammtB 

 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 structures, as well as the strictly rudi- 

 mentary ones, reveal the descent of man from some lower form 

 in an unmistakable manner. 



Correlated VaHatioit. — In man, as in the lower animals, 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. Geoffrey repeatedly insists, are 

 thus intimately connected. Homologous structures are particu- 

 larly 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.'" 

 Professor Schaaffhausen first drew attention to the relation ap- 

 parently 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 spontaneous, for 

 to our ignorance they appear to arise without any exciting cause. 

 It can, however, be shown that such variations, whether consist- 



"^ The authorities for these several statements are given In my 'Va- 

 riation of Animals under Domestication," vol. ii. pp. 320-335. 



