What Is “ Heterosis ” ? L

T IS not my purpose in this paper to present or to discuss the actual basic I causes or the fundamental mechanisms which bring about the increased size, the excessive kinetic energy, the increased productiveness, resistance to disease or to unfavorable conditions of the environment, the “stimulating effects of hybridity,” the so-called “hybrid vigor” of English writers and “Luxurieren” of many German writers, which may be observed in this or that cross-bred organism when compared with corresponding inbred or relatively more pure-bred organisms. My object is merely to consider the spirit and the coverage of the word “heterosis” which I proposed in 1914 to replace the more cumbersome word “heterozygosis” which had been found useful by a few geneticists of that time, myself among them. What I have to say about the meaning of the word “heterosis” will be better understood if it be kept in mind that I was, as far as I know, the first Englishlanguage geneticist to adopt and promote the use of the brilliantly conceived terminological proposals of the Danish plant physiologist, DOCTOR W. JOHANNSEN, which have contributed so much to the precision, clarity and stability of modern genetical terminology. JOHANNSEN’S proposals of “phenotype,” “biotype,” “gene,” “genotype,” etc., were launched in 1909 in his “Elemente der exakten Erblichkeitslehre,” and in that same year his word “biotype” appeared in the title of one of my Carnegie Institution publications, ‘(Bursa &~sa-pastoris and Bama Heegeri; biotypes and hybrids.” In proposing the German word “Gen,” for whose English form “gene” I was more or less responsible, JOHANNSEK pointed out that DARWIN’S word pangene which had come into use as a specific improvement over the older, ambiguous term “Anlage,” was a double expression, consisting of the two parts, “pan” and “gene” of which the first syllable, “pan,” was wholly immaterial and irrelevant, and only the second syllable represented the concept which it was desired to express. The word “pangene” had the further disadvantage that it was closely identified with DAKWIN’S “provisional hypothesis” with its migrating gemmules and their differential assortment during ontogenetic development. By omitting the non-essential half of the word “pangene,” JOHANNSEN arrived at his neat word “Gen” which was thus freed from the Darwinian proposals, and he added the specific stipulation that “das Wort ‘Gen’ ist vollig