VARIATION. XI 



individual differences ; such are variations in the number 

 of organs : organs which are often repeated in an organism 

 are extremely prone to vary in number, and such variations 

 are clearly nearly allied to ordinary individual differences. 

 The effect of altered conditions frequently produces varia- 

 tion by a definite action, certain conditions always pro- 

 ducing similar variation ; such variation is usually more 

 considerable than ordinary individual differences. Varia- 

 tions sometimes arise suddenly, it may be by reversion to 

 an anterior type ; but in most cases it is either extremely 

 difficult or impossible to distinguish between reversions 

 and well-marked variations produced by other causes. 

 Variations are frequently transmitted from generation to 

 generation, and the abnormality sometimes increases, some- 

 times diminishes in intensity in each succeeding generation. 

 The more marked forms of variation are, however, rarer 

 and less truly transmitted than those which exhibit less 

 considerable deviation from the usual type. 



The following principal groups of malformations have 

 been included under this Class : 



1. Variations in the position of parts. Situs mutatus, 

 or transposition of the viscera, a condition already ad- 

 verted to. 



2. Variations in the number of parts which are normally 

 repeated several times, such as cases of an additional or 

 deficient rib, or pair of ribs, or vertebra ; or, in plants, of 

 additional or deficient carpels, stamens, &c. 



The various forms of polydactylism have also been in- 

 cluded under this Class, although some are evidently very 

 different to others in their nature. Any division founded 

 on the few specimens which are available would, however, 

 have been far too hazardous : it has therefore been thought 

 best to place all the specimens in one group ; and as the 

 greater part may probably be looked upon as variations, 

 the whole have been included in this Class. 



It is a well-known law that multiple organs are more 

 prone to vary in number than those which are not repeated. 



