SPONTANEOUS VARIATIONS 49 



at the same time varying from the parents. The occasional 

 occurrence of female plumage on a male bird is certainly a 

 variation due to sexual reproduction. Even if we cede the 

 point at issue and agree to restrict the term to characters 

 which neither parent possesses and which are not blends of 

 the characters they do possess, we still have ample evidence 

 that bi-parental reproduction is a cause of variation. For 

 example, if we mate a Burchell zebra which has very broad 

 conspicuous stripes with a pony which has no stripes, the 

 offspring is born with twice as many stripes as the zebra. 1 

 Again, if we cross-breed two perfectly distinct varieties of 

 domestic pigeons for example a pouter and a fantail 

 neither of which has a trace of blue about it we are apt to 

 obtain as offspring a bird which closely resembles the ancestral 

 wild blue rock. In all cross-breeding between allied varieties 

 and species the ancestral type is apt to appear. In all these 

 cases bi-parental reproduction is the necessary antecedent to 

 the appearance of traits in the offspring which neither parent 

 exhibited. Clearly, then, it is a cause of variation. It is true 

 that here we have no evidence of progressive variation ; that 

 is, of variation on which a divergence and an advance on the 

 ancestral type may be founded. But no one has as yet 

 suggested that the term should be so used as to exclude 

 regressive changes. 



83. On the other hand we have now ample evidence that 

 sexual reproduction is not the sole cause of variations. This 

 fact was long ago demonstrated conclusively by Professor 

 Vines. Writing of the Basidiomycetes he stated 



" These Fungi are not only entirely asexual, but it would 

 appear that they have been evolved in a purely asexual 

 manner from asexual ascomycetous or secidiomycetous 

 ancestors. The Basidiomycetes, in fact, afford an example of 

 a vast family of plants, of the most varied form and habit, 

 including hundreds of genera and species, in which, so far as 

 minute and long-continued investigation has shown, there 

 is not, and probably never has been, any trace of a sexual 

 process." 2 



84. The Basidiomycetes could not have undergone differen- 

 tiation without the occurrence of variations. More recently 

 Professor Weismann himself discovered evidence of variations 

 occurring in the absence of sexual reproduction. " Two 

 varieties of a small Ostracod (Cypris reptans), possessing 

 a very marked coloration, occur in certain ponds in the 



1 See The Penicuik Experiments. 



2 Nature, vol. xl., p. 626. 



