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Show 338 LAWS OF VARIATION. CHAP. xxv. hair of any other colour, but. that in additi~n so~e great con~ stitutional difference must stand in correlatiOn w1th the c~lom of the hair ; for in the above-mentioned cases, vegetable pOisons caused fever, swelling of the head, as well as othe~· symptoms, and even death, to all the white or white-spotted ammals. CIIAJ'. XX\'I. AFFINITY OF IJOMOLOGOUS PARTS. 339 CHAP'rER XX VI. J,A WS 01<' VARIATION, continued - SUMMARY. ON THE AFFINITY AND OOHESIO~ 01<' HOMOLOGOUS PAHTS- ON '.rHE VARIABILITY OF llfULTIPLE AND HOMOLOGOUS PAR'.rS- 00i}IPENSA'l'ION OF GROWTII- llfEOHANIOAL PRESSUUE- RELA'l'IVE POSI~'ION OF FLOWERS WITH HESPEOT 'T'O THE AXIS OF THE l'LANT, AND OF SEEDS IN THE OAPSCLE, AS Di'DUOING VARIATION- ANALOGOUS OR l'ARALLEL VARIETIES- SUliDtARY OF THE TIIREE LAS1' CHAPTERS. On the Affinity of Homologous Parts.-Turs law was first generalised by Geoffroy Saint Hilaire, under the expression of La loi de l'affinite de soi pour soi. It has been fully discussed and illustrated by his son, Isidore Geoffroy, with respect to monsters in the animal kingdom/ and by 1\Ioquin-Tandon, with respect to monstrous plants. When similar or homologous parts, whether belonging to the same embryo or to two distinct embryos, are brought during an early stage of development into contact, they often blend into a single part or organ; and this complete fusion indicates some mutual af-finity between the parts, otherwise they would simply cohere. \Vhether any power exists which tends to bring homologous parts into con tact seems more doubtful. The tendency to complete fusion is not a rare or exceptional fact. It is exhibited in the most striking manner by double monsters. Nothing can be more extraordinary than the manner, as shown in various published plates, in which the corresponding parts of two embryos 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 union of two half- 1 'Hist des Anomalies,' 1832, tom . i. pp. 22, 537-!\56; tom. iii. p. 462. . 7, 2 |
