Chap. XXVI. FUSION OF HOMOLOGOUS FARTS. 333 



CHAPTER XXVI. 



LAWS OF VARIATION, continued SUMMARY. 



1HE FUSION OF HOMOLOGOUS PARTS — THE VARIABILITY OF MULTIPLE AND 

 HOMOLOGOUS PARTS — COMPENSATION OF GROWTH — MECHANICAL PRESSURE 



RELATIVE POSITION OF FLOWERS WITH RESPECT TO THE AXIS, AND OF 



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

 VARIETIES — SUMMARY OF THE THREE LAST CHAPTERS. 



TJie Fusion of Homologous Parts. — Geoffroy Saint-Hilaire 

 formerly propounded -what he called la loi de Vaffinite de soi 

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

 Isidore, with respect to monsters in the animal kingdom, 1 

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

 This law seems to imply that homologous parts actually 

 attract one another and then unite. No doubt there are 

 many wonderful cases, in which such parts become intimately 

 fused together. This is perhaps 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, 2 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 capable judges that the homolo- 

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

 of Air. Lowne : 3 "As union takes place before the differentia- 



1 ' Hist, des Anomalies,' 1832, torn. 3 ' Catalogue of the Teratological 

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



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

 1029. 



