XVI.] THE LAWS OF VAKIATION. 201 



is SO great, and the intermediate forms present such 

 innumerable gradations^ that it is utterly impossible to 

 fix the species.^ 



" When any part or organ is repeated many times in the Parts 

 structure of the same individual, as the vertebrae in snakes ^<^psated 



many 



and the stamens in polyandrous flowers, the number is times are 

 variable ; whereas the number of the same part or organ, ^^"'^ ^' 

 when it occurs in lesser numbers, is constant." '^ 



Between these two laws — that low organisms are vari- 

 able, and that the number of often-repeated parts is 

 variable — there is this connexion, that multitude of similar 

 parts is a mark of low organization. And we may assign Reason of 

 this very obvious reason for both, that the lower the orean- }^^^^^ *^^° 



1 -n • p laws, 



ism, the more wul its form be a matter of indifference ; 

 and the greater the number of parts, the more will their 

 exact number be a matter of indifference. The form of a 

 sea-weed must be nearly a matter of indifference to its 

 health and life ; but any great deviation from the normal 

 form in one of the higher animals constitutes deformity, 

 and is destructive. And, similarly, a pair of legs more or 

 less may make no difference to a centipede, but it is an 

 affair of life or death to a quadruped. 



The laws of ordinary spontaneous variation, as stated Summary. 

 above, may be thus summed up : — 



Spontaneous variations are quite distinct from func- 

 tionally-produced modifications. Spontaneous variation 

 occurs only when a new individual comes into existence. 

 It occurs oftener in cases of sexual than of non-sexual 

 propagation, and is stimulated by mixture of races and by 

 changes in the external circumstances of life. 



Some races are more variable than others ; and when a 

 race has become variable, some characters are more variable 

 than others. When a race, or a character, begins to vary, 

 it will continue to vary for an indefinite time. 



Homologous parts tend to vary together, and there are 

 other correlations which cannot be reduced to any law. 



1 Carpenter on the Foraminifera, published by the Eay Society, 1862. 

 * Darwin's Origin of Species, p. 176. 



