Jhap. II. Manner of Development. 43 



That this rinknown factor is reversion to a former state of 

 Bxistonoe may be admitted as in the highest degree probable." 

 It is quite incredible that a man should through mere accident 

 abnormally resemble certain apes in no less than seven of his 

 muscles, if there had been no genetic connection between them. 

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

 creature, no valid reason can be assigned why certain muscles 

 should not suddenly reappear after an interval of many thou- 

 sand generations, in the same manner as with horses, asses, and 

 mules, dark-coloured stripes suddenly reappear on the legs, 

 and shoulders, after an interval of hundreds, or more probably 

 of 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 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 whicli are rudi- 

 mentary in man, as the os coccyx in both sexes, and the iiiamiiia) 

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

 supracondyloid foramen, only occasionally appear, and thero!bre 

 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 Variation. — 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 



possibility of either of his first pro- closely the variations resemble the 



positions. Prof. Macalistcr has also normal muscles of the lower ani- 



de.scribed (' Proo. R. Irish Acad.' mats. He sums up by remarlcing, 



vol. X. 1864, p. 138) variations in " It will be enough for my purpose 



the yZexor- poWcj's tajtto, remarkable "if I have succeeded in r.hewing 



from their relations to the same " the more important forms which, 



muscle in the Quadrumana. " when occurring as varieties in the 



^-i Since the first edition of this " human subject, tend to eshibit ia 



book appeared, Mr. Wood has pub- " a sufficiently marked manner what 



lished another memoir in the ' Phil. " may be considered as proofs and 



Transactions,' 1870, p. 83, on the " examples of the Darwinian prin- 



rarietiesofthe muscles of the human " ciple of reversion, or law of ic. 



leck, shoulder, and chest. He here " heritance, in ^ this department rt 



ohows how extremely variable these •' .inatomical science." 

 muscles are, and how often and how 



