JEFFREY: EVOLUTION BY HYBRIDIZATION 299 
contented himself mainly with pointing out that a general process of 
variation has been going on from age to age in matter endowed with 
life. He emphasized the fact that the struggle for existence on the 
one hand and the selection exercised by environment on the other 
provided an important directing influence upon the development of 
new species of plants and animals. In recent years a doctrine of old 
standing has been revived, namely the hypothesis of mutation. It has 
been maintained that new forms or elementary species arise spon- 
taneously from formerly existing species. This doctrine has been 
particularly advanced by the activities of the Dutch physiologist De 
Vries and his disciples in this and other countries. It is a general 
observation in connection with the activities of the lower organisms 
that in the process of their often extremely active development they 
give rise to inhibiting substances. In the case of the common yeast 
for example we have the formation of alcohol, which finally, by a high 
degree of concentration in fermenting sugary solutions, brings the 
activity of the yeast organism to a close. It is of interest to note in 
this connection that it is precisely in Holland that scientific opposition 
to the mutation hypothesis of De Vries has recently appeared. To 
Dr. Lotsy we owe a recent volume on Evolution by Means of Hybrid- 
ization, which attacks the mutation hypothesis at its very base 
through the contrary hypothesis that all changes in living matter are 
due to crossing or hybridization and are not the consequence of 
spontaneous internal or mutational phenomena. The author argues 
that since hybrids are notoriously variable all variability must be 
due to hybridism. This appears to be reasoning in a vicious circle. 
Clearly the most definite evidence in regard to hybridism as the cause 
of new species should be demanded before the possibility of the 
appearance of new types in this manner can be admitted. We 
fortunately have extremely good testimony on this subject from the 
earlier investigations published by Kerner in Austria and Brainerd in 
this country. Kerner in his well-known Pflanzenleben as well as in 
an earlier publication in the Oesterreiche Botanische Zeitung has 
brought forward much evidence as to the origin of new species as the 
result of hybridization in the mountainous regions of eastern central 
Europe, where the floras of the Pontic, Mediterranean and Baltic 
areas meet. It is impossible within the time at my disposal to make 
more than a very brief reference to the results reached by this writer. 
He has made it clear that the members of different floras are very 
apt indeed to produce new species by hybridization in nature and that 
these species, where they are advantageously equipped as compared 
with the parent forms, flourish within the same region. In case they 
have qualities which enable them to live where the parental species 
