86 
THE PLANT WORLD. 
easily damaged, such as willow, etc., a three per cent, solution of 
commercial formaldehyd may be used. A solution of formaldehyd 
(i per cent.) and chrome alum (i per cent.) in water is recom¬ 
mended because it does not make the tissues brittle. 
REVIEWS. 
Species and Varieties : Their Origin by Mutation* 
I 
If all the plants and animals now living and that have ever existed in 
the past could be assembled in one place, it would be possible to arrange 
them in such a way as to show their relationship to each other. This 
is the doctrine of descent, and is part of the broader theory of evolution. 
The general acceptance of organic evolution dates from, the appearance of 
the Origin of Species in 1859. 
The process of natural selection was elaborated by Darwin and in less 
detail by Wallace as the method of the origin of species. Thus arises a 
demand for materials upon which natural selection may act. This demand 
is supplied by variation. 
Two kinds of variations are now recognized. Those resulting from, the 
influence of environment, direct or indirect, are termed individual. Since 
all degrees of intermediate grades are recognized between the extremes 
of departure and the norm, this is sometimes called continuous, and since 
the degree of variation is not fixed, but may oscillate about a mean 
between two extremes, the term fluctuating variation is also employed. 
Continuous or fluctuating variation always means an increase or decrease 
of some quality or character already existing. It is a case of more or 
less. A fruit may grow sweeter, a flower larger, a root fleshier, but 
nothing really new results. 
There is also recognized another kind of variation which proceeds by 
the introduction of entirely new characters. Structural features not 
present before result, and what is more important yet, the change is 
abrupt and permanent. There has been a sudden and complete change. 
Transitional forms are wanting. This is discontinuous variation. 
Which kind of variations furnish the material for natural selection to 
act upon? Do both kinds or does only one? For Darwin and his fol¬ 
lowers it was mainly the fluctuations. The serious difficulties in the way 
of,accepting this idea have never been successfully met. According to 
the theory, natural selection is a case of life and death. The struggle 
is between the fit and the unfit, and only the useful survives. But how 
can incipient new characters and their earlier intermediate grades be of 
* Species and Varieties: Their Origin by Mutation. Hugo deVries, 
edited by D. T. MacDougal. Chicago: The Open Court Pub. Co. 1905. 
