I 10 
THE PLANT WORLD. 
REVIEWS. 
Species and Varieties: Their Origin by Mutation* 
II 
Chapter V. makes clear the idea of retrograde varieties. While ele¬ 
mentary species “ arise by the acquisition of entirely new characters,” 
varieties have their origin either “ by the loss of existing qualities or by 
the gain of such peculiarities as may already be seen in closely allied 
species.” “ The elementary species have mutated in the line of progres¬ 
sion, while others have diverged from their parental types on a line of 
retrogression, or in the way of repetition.” The latter are termed “ retro¬ 
grade varieties.” For example, among the composites there is a group of 
genera, among which the daisy, yarrow, and aster are examples, having 
both disc and ray florets. Another group lacks the ray florets, e. g., the 
tansy, and some artemisias. Among the daisies and asters, however, some 
forms are found in which the ray florets are lacking. These “ discoid ” 
forms are retrograde varieties. Among the tansies, on the other hand, 
ray flowers are sometimes observed. These “ radiate ” forms show char¬ 
acters already existing in closely allied species, and are progressive varie¬ 
ties. Sugar corn is a retrograde variety resulting from loss of starch in 
the kernel, and its replacement by sugar or some allied substance. Here 
also belong weeping willows and other weeping trees, which have lost 
the power of upright growth of the branches. 
Atavism .—Oftentimes plants revert to some supposed prototype. Re¬ 
version of this kind is atavism. It is usually impossible to prove that the 
type towards which reversion has taken place is the parent species. We 
have ordinarily only indirect evidence on this point. If we could have 
a plant with a pedigree and it should occasionally revert to an ancestral 
form, then we shall have a case of real atavism. Before designating such 
a variation as atavism we must be sure that the race is pure, not hybrid, 
otherwise Mendelism, or a splitting of a hybrid generation into its parent 
types, may be confused with atavism. 
Reliable, well-authenticated reversion by seeds is extremely rare, for 
in most of the recorded instances there is grave doubt as to the purity 
of the seeds and the possibility of unobserved crosses. Apparent atavism 
might result also if the supposed prototype were a hybrid. On these 
grounds, bud variations, only, are accepted as direct proofs of true ata¬ 
vism. As examples of true atavism may be mentioned the cut-leaved 
varieties of different plants, as the birch and beech, which often develop, 
from buds, stems having the ancestral character of entire leaves. “ But 
this is merely due to the wide variability of the degree of fissure and is 
to be considered only as a fluctuation between somewhat widely distant 
extremes, which may apparently include the form of the common beech 
* Species and Varieties: Their Origin by Mutation. Hugo deVries, 
edited by D. T. MacDougal. Chicago: The Open Court Pub. Co. 1905. 
