Chap. XXVL FUSION OF HOMOLOGOUS PARTS. 333 



CHAPTER XXVI. 



LAWS OF VARIATION", Continued — summary. 



THE FUSION OF HOMOLOGOUS PARTS — THE VARIABILITY OP MULTIPLE AND 

 HOMOLOGOUS PARTS — COMPENSATION OP GROWTH — MECHANICAL PRESSURE 

 — RELATIVE POSITION OF FLOWEHS WITH RESPECT TO THE AXIS, AND OF 

 SEEDS IN THE OVARY, AS INDUCING VARIATION — ANALOGOUS OR PARALLEL 

 VARIETIES — SUMMARY OF THE THPvEE LAST CHAPTERS. 



TJie Fusion of Homologous Parts. — Geoffroy Saint-Hilaire 

 formerly propounded what he called la loi de Vaffiniie de soi 

 pour soi, which has been discussed and illustrated by his son, 

 Isidore, with respect to monsters in the animal kingdom,^ 

 and by Moquin-Tandon, with respect to monstrous plants. 

 'I'his law seems to imiily that homologous parts actually 

 attract one another and then unite. Xo doubt there are 

 many wonderful cases, in which such parts become intimately 

 fused together. This is perhaj)S best seen in monsters with 

 two heads, which are united, summit to summit, or face to 

 face, or Janus-like, back to back, or obliquely side to side. 

 In one instance of two heads united almost face to face, but a 

 little obliquely, four ears were developed, and on one side a 

 perfect face, which was manifestly formed by the fusion of 

 two half-faces. Whenever two bodies or two heads are 

 united, each bone, muscle, vessel, and nerve on the line of 

 junction appears as if it had sought out its fellow, and had be- 

 come completely fused with it. Lereboullet,^ who carefully 

 studied the development of double monsters in fishes, observed 

 in fifteen instances the steps by which two heads gradually 

 became united into one. In all such cases it is now thought 

 by the greater number of ca|)able judges that the homolo- 

 gous parts do not attract each other, but that in the words 

 of Mr. Lowne : ^ "As union takes jjlace before the difierentia- 



^ * Hist, des Anomalies,' 1832, torn. ' ' Catalogue of the Teratological 



L pp. 22, 537-556 ; torn. iii. p. 462. Series in the Museum of the R. Coll. 



^ • Comptes Rendus,' 1855, pp. 855, of Surgeons,' 1872, p. xvi. 

 1029. 



