Chap. XXVI. AFFINITY OF HOMOLOGOUS PARTS. 



CHAPTEK XXYI. 



LAWS OF VARIATION, continued — SUMMARY. 



ON THE AFFINITY AND COHESION OF HOMOLOGOUS PAETS — ON THE VARIABILITY OF 

 MULTIPLE AND HOMOLOGOUS PAETS — COMPENSATION OF GROWTH — MECHANICAL 

 PRESSURE — RELATIVE POSITION OF FLOWERS WITH RESPECT TO THE AXIS OF THE 

 PLANT, AND OF SEEDS IN THE CAPSULE, AS INDUCING VARIATION — ANALOGOUS OR 

 PARALLEL VARIETIES — SUMMARY OF THE THREE LAST CHAPTERS. 



On the Affinity of Homologous Parts. — This law was first 

 generalised by Geoffrey Saint Hilaire, under the expression of 

 La hi de VaffiniU de soi pour soi. It has been fully discussed and 

 illustrated by his son, Isidore Geoffrey, with respect to monsters 

 m the animal kingdom, 1 and by Moquin-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 affinity between the 

 parts, otherwise they would simply cohere. Whether 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 

 sinking manner by double monsters. Nothing can be more ex- 

 traordinary 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 



2vZT7 W ° hGadS United almost &ce 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 ' H1St ^ An ° ffialieS '' 1832 > tan. i. PP. 22, 537-556 ; torn. iii. p. 462. 



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