REVIEWS. 
89 
must sharply distinguish between progression and retrogression, for pro¬ 
gressive steps mark an elementary species. This process is mutation , 
and the product a mutant (elementary species).* 
Mutants make a clear departure from their ancestors. They are not 
connected with them by gradations, and do not revert to the parent type. 
Fluctuating* variations “ always oscillate around an average, and if 
removed from this for some time they show a tendency to return to it ” 
(p. 18). This tendency is retrogression. 
Mutants, as well as fluctuations, furnish the material for both natural 
and artificial selection to work upon. There is both an inter-specific and 
an intra-specific, or individual struggle. The battle between the indi¬ 
viduals of a species leaves a hardier or “ fitter ” species, that between 
species allows only the hardiest or fittest species to survive. Thus Spen¬ 
cer’s expression, “ survival of the fittest,” as Morgan has pointed out, 
is incomplete and should read, “ survival of the fittest species.” 
Not all plants are now in a state of mutation, but for experimentation 
it is necessary to find a plant which is in the mutative period. This 
deVries succeeded in doing in the case of Lamarck’s evening-primrose 
(Oenothera Lamar ckiana ). The most important point in experimenting 
is to know accurately the pedigree of the plants used. Elementary species 
are found in nature, but their history is not known and their origin was 
not observed. 
Since Linnaeus species have been considered the units of nature. Pre¬ 
vious to his time the genera were the units, and nearly all the common 
names of plants—oak, clover, rose, refer to genera. Now we must recog¬ 
nize the fact that the real units are the elementary species. 
The species of the systematists are nearly always compound groups 
containing from two to fifty “ or even hundreds of well differentiated 
forms.” Some authors recognize more of these elementary forms than 
do others. Bentham and Hooker, for example, describe the bramble under 
five species, while Babbington makes forty-five species out of the same 
material. Drab a verna comprises over two hundred elementary species 
(P- Si)- 
Among cultivated plants elementary species are found in the beets, 
apples, pears, clover, flax, cocoanut, wheat, oats and others. Wild apples 
are at present very rich in elementary species. All cultivated plants came 
from wild ancestors, and different elementary species among these wild 
forms gave rise to different types of the cultivated plants. Of course 
artificial selection of individual variations has been one of the chief factors 
in producing the different varieties. 
Two kinds of breeding selection, first distinguished by Von Rumker, 
must be recognized. 
1. Production of new forms. These are produced by nature without 
* New elementary species also result from crossing, but this part of the 
theory, elaborated in Vol. 2 of the Mutationstheorie, is not treated of in 
the English work. 
