Chap. IV.] MANNER OF DEVELOPMENT. ]25 



suddenly reappear after an interval of many thousand 

 generations, 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 prob- 

 ably thousands, of generations. 



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 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 mammoe in the male 

 sex, are always present ; while others, such - as the supra- 

 condyloid foramen, only occasionally appear, and there- 

 fore might have been introduced under the head of rever- 

 sion. These several reversionary, as well as the strictly 

 rudimentary, structures 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 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 op- 

 posite sides of the body, and in the upper and lower ex- 

 tremities. 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 



