330 SIGNIFICANCE OF SEXUAL REPRODUCTION [V. 



between these two extremes all intermediate 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 ^ 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 combination of ancestral characters a new character 

 (bare-headednessj 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 which is composed of a countless number of ancestral germ- 

 plasms from the first and second back to the ;^th degree. But 

 we can nevertheless maintain in a general way that such ir- 

 regularities are the result of this complex struggle between the 

 germ-plasms in the ovum and the idioplasms in the subsequent 

 stages of the developing organism, and that they are not the 

 result of external influences. 



If, however, acquired characters are brought forward in con- 

 nexion with the question of the transformation of species, the 

 term ' acquired ' must only be applied to those characters which 

 do not arise from within the organism, but which arise as the 

 reaction of the organism under some external stimulus, most 



^ See Darwin, 'The Variation of Animals and Plants under Domesti-. 

 cation.' 1875. Vol. I. p. 311. 



