166 HISTORY OF THE HUMAN BODY 



.complete reduction of both pairs is correlated with an exces- 

 sive lengthening of the body, locomotion being effected by 

 .an undulatory movement of the entire animal. In snakes, 

 .which progress mainly through the action of their very 

 numerous ribs, the loss of both pairs of limbs is usually 

 a total one, but in the boas the posterior limbs ap- 

 pear as spur-like rudiments, situated upon either side of the 

 cloacal orifice, and are of considerable use in climbing trees. 

 Aside from the snakes a similar form is assumed by several 

 groups of reptiles and amphibians, the adaptation fitting them 

 in some cases for a life similar to that of snakes, and in others 

 for a subterranean existence. These latter, which include at 

 least one group of lizards and one of amphibians, burrow in 

 the earth like earth-worms, and as in the process of this 

 adaptation they have lost their eyes, reduced their head and 

 arranged their scales in the form of annular segments, they 

 resemble these latter animals almost to the point of deception. 

 Through all their vicissitudes, however, the number of free 

 limbs is constant in all vertebrates, except when secondarily 

 reduced, and consists of two pairs, corresponding, as we sup- 

 pose, to the number of original points at which the primitive 

 fin-fold became hypertrophied. Although this number may 

 be reduced as a special adaptation, it can never be increased, 

 and the favorite mythological conceptions of human and other 

 vertebrate forms with supernumerary limbs are far more im- 

 possible and absurd than is usually recognized, since they are 

 generally held to be merely contrary to experience, but are 

 here seen to violate the fundamental principles of develop- 

 ment.* It might, indeed, have been possible in the first place 

 for the lateral fin-folds to have hypertrophied in three or more 

 places instead of two, a result which some slight change of 

 environment or habit would have then easily effected, but the 



* Cases of monstrosities with extra limbs, in which the total number ex- 

 ceeds four, are not violations of this principle, but are anomalies due to a 

 multiplication of certain of the anlagen, like monsters with two heads or 

 two bodies. The cause of such redundancy is as yet imperfectly under- 

 stood, but enough has been already proven to show that it lies in the 

 germ, in which the abnormality probably exists in the form of redundant 

 germinal elements. 



