IN THE THEORY OF NATURAL SELECTION. 321 



germ-plasms which takes place at fertilization, the contained an- 

 cestral germ-plasms unite in different ways, and thus come to grow 

 with different strengths. Certain ancestral germ-plasms will meet 

 and together produce a double effect : other opposed germ-plasms 

 will neutralize each other ; and between these two extremes all in- 

 termediate conditions will occur. And these combinations will not 

 only take place at fertilization, but also at every stage of the whole 

 ontogenetic history, for each stage is represented by its idioplasm, 

 which is itself composed of ancestral idioplasms. 



We do not yet know enough to be able to prove in detail 

 the manner in which new characters may arise from such a com- 

 bination of different kinds of germ-plasm. And yet it appears to 

 me that such a view, e. g. in the case of the variation of buds, is by 

 far the most natural. There is indeed a single example in which 

 we can, to some extent, understand how it is that a new character 

 may arise by these means. Certain canary-birds have a tuft of 

 feathers on the head, but if two such birds are paired, their 

 descendants are generally bare-headed, instead of having larger 

 tufts 1 . The formation of a tuft depends upon the fact that the 

 feathers are scanty and in fact absent from part of the skin of the 

 head. Now when the scanty plumage of both parents is combined 

 in the offspring the latter is bare-headed. Hence by the com- 

 bination of ancestral characters a new character (bare-headedness) is 

 produced, and one which is hardly likely to have ever occurred in 

 the ancestors of existing canaries. 



We do not know the causes which have been in operation when 

 a flower possesses one petal more than the usual number, any more 

 than we can explain why it is that one star-fish has five and 

 another six rays. We cannot unravel the details of the mysterious 

 relationship between two parent germ-plasms, each of w T hich is 

 composed of a countless number of ancestral germ-plasms from the 

 first and second back to the nth degree. But we can neverthe- 

 less maintain in a general way that such irregularities are the 

 result of this complex struggle between the germ-plasms in the 

 ovum and the idioplasms in the subsequent stages of the de- 

 veloping organism, and that they are not the result of external 

 influences. 



1 See Darwin, 'The Variation of Animals and Plants under Domestication.' 1875. 

 Vol. I. p. 311. 



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