I 
90 
THE PLANT WORLD. 
man’s aid, and “ have only to be selected and isolated, and their progeny 
at once yields a constant and pure race.” 
2. Race improvement. This is the work of man. Nothing new is 
created, but what already exists is developed and improved. Progression 
is counteracted and the race brought above the average. This is the 
process commonly designated as variety testing. 
This discussion serves to illustrate again the fact that natural selection, 
operative through the effects of climate, soil and water supply, does not 
create anything new, but acts as a sieve, conserving the fittest species and 
letting the unfit perish. Discontinuous variation is evolution. Natural 
selection makes permanent the victory won by the newly evolved form. 
Mutation produces the combatants in the struggle for existence and the 
result of the conflict is the survival of the fittest species. 
C S. G. 
(To be continued .) 
Organic Evolution,* by Professor Metcalf, embodies the author’s lec¬ 
tures of this subject to the students of the Woman’s College of Baltimore. 
We learn from the preface that the writer has endeavored “to avoid 
technicality as far as possible, and present the subject in a way that will 
be intelligible to those unfamiliar with biological phenomena.” This aim 
has been successfully accomplished. The book is beautifully illustrated, 
partly by colored plates. Possibly too much dependence has been placed 
upon the illustrations. They are fully explained, but one seems to need 
the lecturer with pointer to make their relation with the text more inti¬ 
mate. The book will prove to be interesting and instructive reading, 
especially for those for whom is it primarily intended. It is by far the most 
attractive popular presentation of the theory of organic evolution. We ven¬ 
ture the prediction, however, that future works on this subject, whether 
technical or “ popular,” will devote less space to the debatable ground of 
protective coloration, mimicry and closely allied topics, and more to 
variation and the experimental study of the origin of species. It is with 
these questions that the next generation of biologists will be chiefly con¬ 
cerned. The appendix contains valuable suggestions for a course of 
reading in “ organic evolution and phenomena of special adaptation.” 
C. S. G. 
* An outline of the Theory of Organic Evolution, by Maynard M. 
Metcalf, pp. xxii and 191. The Macmillan Co. 1904. 
