TWINNING IN BIRDS 75 



agreed with Winslow's objections to the idea of fusion 

 and proposed the theory that double monstrosity was 

 determined through some pecuHarity of the process of 

 fecundation. 



Meckel, who was a follower of Wolff, carried the 

 epigenesis conception still farther and concluded that 

 all double monsters are cases of developmental excess, 

 inasmuch as they are derived from a single egg. All 

 supernumerary parts are conceived of as the result of 

 a complete doubling of a particular organ. This doub- 

 ling might involve only one finger or the whole body, as 

 in the case of twins. He believed in a dual origin of a 

 bilaterally symmetrical animal, and that in double 

 monsters the two halves fail to unite or unite only 

 partially. Like Wolff he sought to explain the doubling 

 as the result of a peculiarity of the process of fecundation 

 such as double fertiUzation or some other irregularity. 



In 1826 Etienne Geoffroy Saint-Hilaire revived the 

 fusion idea of Lemery, but instead of supposing that the 

 fusion was a purely accidental phenomenon, . he tried to 

 explain the striking symmetry of monsters by stating 

 that homologous organs have an affinity for each other 

 and only like parts would fuse with like. Saint-Hilaire 

 did not follow up this hypothesis but passed it on to his 

 son, who developed the idea much farther. The latter 

 urged against the theory of Meckel such cases as double 

 pelvis, double breasts, double faces, which, however, 

 seemed to him easily explicable on the basis of the 

 hypothesis of original duality. 



So influential was the fusion theory of the two 

 Saint-Hilaires that it has been adopted by Dareste, by 

 Gemmill, and by Stockard. 



