REVIEWS. 
135 
water supply, room, etc.), and “very little on selection.” Atavists occur 
in this race as in the polycephalous poppies, with intermediate gradations 
between them and the wholly twisted variety, but, as in the case of the 
striped flowers of the snapdragon, the series of gradations between the 
atavists and the type of the race is never complete. The gap is never 
entirely bridged over. 
The character of twisted stem is not a variation in the ordinary sense 
of the term. “ It is more a contention of opposite characters,” and we 
have here an instance of a “ double race, including two types which may 
be combined in different degrees.” 
The percentage of cases in which the varietal attribute is transmitted 
is more important than the degree to which it may be possessed by a few 
individuals. This proportion is the hereditary percentage. 
Double Adaptations. —An instance of ever-sporting varieties is repre¬ 
sented in those species which have double adaptations, e. g., the water 
persicaria ( Polygonum amphibium). The terrestrial form (var. terres- 
tris ) and the aquatic form (var. natans) readily sport into each other, 
and the change may even be brought about artificially by transplanting 
one into the environment of the other. Branches of the same plant have 
even in some cases been described by systematists as different species. 
The climbing fig ( Ficus repens), which bears no fruit, and the fruiting 
shrub ( F. stipulata) “ are simply branches of the same plant.” Bonnier 
divided an alpine plant and transferred one half, with plenty of mountain 
soil, to the valley at Paris. The difference in environment made enough 
difference in the two halves to stamp them as varieties. 
Double adaptation is universal in the whole plant kingdom, e. g., the 
response to light and darkness. In ever-sporting varieties two types are 
always present, but one is apt to be useful to the plant, another less so 
and possibly injurious. Thus natural selection which seizes upon only 
useful characters, will usually perpetuate only one type. This explains 
why we do not have, for example, a pure and common race of twisted 
teasels. 
Monstrosities and dimorphous wild species offer the most striking ex¬ 
amples of the inheritance of latent characters. It is instructive to con¬ 
sider in the light of Lamarck’s theory the direct effect of external influ¬ 
ences on organisms in producing new species. If environment results 
only in certain characters becoming latent, they may resume activity even 
after thousands of years of latency, and thus no change of specific value 
is wrought by the direct influence of environment. This conclusion is 
confirmed by Haltermann’s experiments, showing that desert characters 
possessed for centuries disappear easily and quickly when the plants are 
grown in the garden in the absence of their desert environment. 
Mutations. 
The preceding discussion has brought out the idea of unit-forms, 
sharply delimited, each type manifesting fluctuating variability between 
extremes. Extremes of different types may often overlap, but their off¬ 
spring always return to the type. These unit-forms, the elementary spe- 
