GENERAL CONSIDERATIONS AND CONCLUSIONS 439 
species of the Nightshade Family, he actually discovered, according to 
Blaringhem, the essential principle of Mendelism, which he expressed as 
follows: “La disjonction des deux essences specifiques a liew dans le pollen 
et dans les ovules de Vhybride.”” To Nageli we are indebted for the first 
scientific treatise on hybridization that was wholly impartial and coherent. 
His works served as a common source for most of the later discussions 
of plant hybrids. Meanwhile Darwin had organized a great mass of 
information bearing on the general subject of adaptation and had dis- 
covered one general principle of evolution, viz., the principle of natural 
selection. The publication of Darwin’s discovery was at once stimulating 
and deterring. It was stimulating to argumentative controversy as well 
as to certain students of heredity, notably Hoffman, who conducted nu- 
merous experiments from 1855 to 1880, and Galton, whose work during the 
80’s laid the foundation for the biometrical method of treating the data 
of genetics. Darwin’s theory was deterring in its effect on a further wid- 
ening of the biological horizon, at least so far as the theory of evolution 
was concerned. Biologists were so well satisfied with his conception 
that all minute, fluctuating variations are inherited and so may be pre- 
served by natural selection, that but little real progress was made in the 
study of evolution until the rediscovery of Mendel’s discovery in 1900. 
The work of Mendel, although unappreciated by Nageli and other con- 
temporaries who knew of it, was destined to revolutionize the study of 
heredity. By his critical experiments and keen interpretations of the 
results of those experiments, Mendel laid the cornerstone of the foun- 
dation for the future science of genetics. During the 19th century, 
plant and animal breeding was in progress in various countries and the 
reported observations of many experimentalists on a multitude of living 
forms presented an array of diverse and apparently contradictory phe- 
nomena, the classification of which under a few natural laws was hardly 
considered. 
With the announcement of the discoveries of Mendel, de Vries and 
Johannsen during the first three years of the present century, there was 
a great awakening of interest among biologists in the problems of varia- 
tion, heredity and evolution. In the enthusiasm of the hour it was 
thought by some that the application of these laws of heredity and muta- 
tion in practical breeding would be comparatively a simple matter. 
Many plant breeders went zealously to work only to obtain further con- 
flicting and disconcerting results. At the same time research students 
the world over began new investigations on variation and heredity. 
The occurrence of mutations and the existence of pure lines in species 
that reproduce by self-fertilization only have been verified; but to ex- 
plain the heredity of most plants requires certain modifications or ex- 
tensions of the three original Mendelian “laws.” The investigations 
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