REVIEWS. 
I I I 
leaves. It is not a bud-variation at all, and it is to be met with quite 
commonly while the true reversions by buds are very rare and are of 
the nature of sports appearing suddenly and remaining constant on the 
same twig,” p. 179. 
Vicinism. —The rare phenomenon of “ true atavism or reversion caused 
by an innate tendency,” must be carefully distinguished from “false ata¬ 
vism,” which results from crossing with neighboring plants. This is the 
“ atavism ” of the breeders, but since it is quite a different process from 
true atavism, it is proposed to give to it the term “vicinism” (Latin 
vicinus, neighbor). The fact of vicinism as distinguished from true ata- 
ism has been established by different experiments with Aster Tripolium, 
creeping thyme, and white self-heal. 
Latency. —A common phenomenon of plant life is latency, or the lying 
dormant of one or more characters through several generations, with 
subsequent reappearance. There may be the ordinary type, individual 
latency, which may show itself from time to time during the life history 
of an individual, as when the characters of seed leaves, or cotyledons, 
die latent in the tissues of the plant from germination to the develop¬ 
ment of the next embryo^ or when willow and oak trunks produce shoots. 
We may also recognize systematic latency, manifest only at intervals in the 
lifetime of the species or variety This has no regular periodicity and may 
not be excited by artificial means. True atavism is a case in point. 
“ Wheat ear ” carnations, and green dahlias revert, not to the charac¬ 
teristics of the ordinary species, but to the particular variety from which 
they have sprung, showing that these varietal qualities must have been 
present in the wheat-ear plants (and in the green dahlias), but latent. 
The facts of latency have a direct bearing on the question of the differ¬ 
ence between species and varieties. Negative, or retrogressive, varieties 
result when some character of the species becomes latent— e. g., white 
varieties of colored flowers. “ Positive varieties on the contrary owe 
their origin to the presence of some character in the species in the latent 
state, and to the occasional re-energizing thereof,” e. g., colored or tinted 
varieties of a white species. In such a case it is nearly always the rule 
that the color or tint of the variety occurs in other species of the genus, 
and the inference is that it was latent and only apparently lacking in the 
white species. 
A systematic species is composed of elementary species which are of 
equal rank and not derived from one another, while varieties of a species 
are “ derived from real and still existing types.” The method by which 
elementary species arise is termed “progressive evolution,” in contrast to 
“retrograde evolution,” which proceeds by the loss of latency of charac¬ 
ters, and regressive (degressive ) evolution, where a latent character reas¬ 
sumes its activity and returns to the active state (p. 221). 
In all three of the above cases the important fact is that “ the differen¬ 
tiating marks show more or less clearly that they are made up of units.” 
“Transitions are wholly wanting, although fallaciously apparent in some 
instances owing to the wide range of fluctuating variability of the forms 
concerned, or to the occurrence of hybrids and subvarieties.” “ These 
