VARIATION STEPS 195 



From another standpoint the differences between the prim- 

 ordia under consideration may be expressed by measurement. 

 The rose colour appears at a certain moment, let us say after n 

 hours ; the blue colour after n' hours. A petal stopped in its 

 development before the moment n is white ; if stopped before 

 the moment n' it is rose ; if stopped in its development after n' 

 it is blue. The curve of development being constructed, the 

 differences may be reduced to the terms of time.^ 



§ 135.— PARALLEL VARIATION.— The term analogous 

 or parallel variation has been used by DARWIN to indicate 

 that similar properties occasionally make their appearance in 

 several " varieties " or " races " descended from the same 

 species and more rarely in the offspring of widely distinct 

 species. DARWIN points out that the majority of the observed 

 cases (occasional appearance of black wing bars in the various 

 breeds of pigeon, and of stripes on the legs of the ass and of 

 various races of the horse, etc.) are evidently due to reversion. 

 Other examples are regarded by many biologists (for instance, 

 by H. M. VERNON, loc. cit., p. 96) as being probably mere 

 coincidences and of no scientific value. 



EXAMPLES (mentioned by DARWIN) ^ : many trees be- 

 longing to quite different orders have produced pendulous and 

 fastigiate varieties. A multitude of plants have yielded 

 varieties with deeply cut leaves. Several varieties of melon 

 resemble other species in important properties. Thus one 

 variety has fruit so like, both externally and internally, the 

 fruit of the cucumber, as hardly to be distinguished from it. In 

 animals we find feather-footed races of the fowl, pigeon and 

 canary bird. Horses of the most different races present the 

 same range of colour. Many " sub- varieties " of the pigeon 

 have reversed and somewhat lengthened feathers on the back 

 parts of their heads, and sub- varieties of the fowl, turkey, canary, 

 duck and goose all have either top-knots or reversed feathers 

 on their heads. It is a common circumstance to find certain 

 coloured marks persistently characterizing all the species of a 

 genus, but differing much in tint. The nectarine is the off- 

 spring of the peach, and the varieties of peaches and nectarines 

 offer a remarkable parallelism in the fruit being white, red or 

 yellow fleshed ; in being cling- stones or freestones ; in the 

 flowers being large or small ; in the leaves being serrated or 

 crenated, furnished with globose or reniform glands, or quite 

 destitute of glands. 



At least three different groups of facts are confounded under 



» Or to the terms of the value of a leading property (§ 49). 

 « See CH. DARWIN, The Variation of Animals and Plants under Domestica- 

 tion, second edition, 1875, vol. ii., p. 340. See also VERNON, loc. cit., p. 96. 



