uIIAl'. 



II. 



Manner of Development. 



43 



That this unknown factor is reversion to a former state of 

 existence may be admitted as in the highest degree probable. 54 

 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-like 

 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 which are rudi- 

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

 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 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- 

 positions. Prof. Macalister has also 

 described (' Proc. R. Irish Acad.' 

 vol. x. 1864, p. 138) variations in 

 the flexor pollicis longus, remarkable 

 from their relations to the same 

 muscle in the Quadrumana. 



54 Since the first edition of this 

 book appeared, Mr. Wood has pub- 

 lished another memoir in the ' Phil. 

 Transactions,' 1870, p. 83, on the 

 rarieties of the muscles of the human 

 leek, shoulder, and chest. He here 

 shews how extremely variable these 

 cmscles are, and how often and how 



closely the variations resemble the 

 normal muscles of the lower ani- 

 mals. He sums up by remarking, 

 " It will be enough for my purpose 

 " if I have succeeded in shewing 

 " the more important forms which, 

 " when occurring as varieties in the 

 " human subject, tend to exhibit in 

 " a sufficiently marked manner what 

 " may be considered as proofs and 

 " examples of the Darwinian prin- 

 " ciple of reversion, or law of in- 

 " heritance, in this department ol 

 •' .inatomical science." 



