342 LAWS OF VARIATION. Chap. XXVI. 



process of fusion. The tendency in homologous parts to unite 

 during their early development, Moquin-Tandon considers as 



one of the most striking laws governing the production of 

 monsters. It apparently explains a multitude of cases, both in 

 the animal and vegetable kingdoms ; it throws a clear light on 

 many normal structures which have evidently been formed by 

 the union of originally distinct parts, and it possesses, as we 

 shall see in a future chapter, much theoretical interest. 



On the Variability of Multiple and Homologous Pat 



Isidore Geoffroy 6 insists that, when any part or organ 



& „- is 



Compensation of Growth, or B alancement .— This law, as applied 

 to natural species, was propounded by Goethe and Geoffroy St. 

 Hilaire at nearly the same time. It implies that, when much 

 organised matter is used in building up some one part, other 

 parts ^ are starved and become reduced. Several authors, 

 especially botanists, believe in this law ; others reject it. As 

 far as I can judge, it occasionally holds good ; but its im- 

 portance has probably been exaggerated. It is scarcely possible 

 to distinguish between the supposed effects of such compensation 

 of growth, and the effects of long-continued selection, which 



6 « Hist, cles Anomalies,' torn. iii. pp. 4, 5, 6. 





. 



f 



^ peated many times in the same animal, it is particularly 

 liable to vary both in number and structure. With respect to 

 number, the proposition may, I think, be considered as fully 

 established ; but the evidence is chiefly derived from organic 

 beings living under their natural conditions, with which we are 

 not here concerned. When the vertebras, or teeth, or rays in ^ 



the fins of fishes, or feathers in the tails of birds, or petals, 

 stamens, pistils, and seeds in plants, are very numerous, the' 

 number is generally variable. The explanation of this simple 

 fact is by no means obvious. With respect to the variability in 

 structure of multiple parts, the evidence is not so decisive ; but 

 the fact, as far as it may be trusted, probably depends on mul- 

 tiple parts being of less physiological importance than single 

 parts ; consequently their perfect standard of structure has been 

 less rigorously enforced by natural selection. 



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