In this book Morgan first used the word gene. Previously he had used the term "Mendelian unit" or "factor." On the basis of genetic analysis Morgan presented a number of characteristics of genes:

1. A gene could have more than one effect. For instance, insects that had white-eye gene not only had white eyes, but al grew slower and had a lower viability.
2. The effects of the gene could be modified by external conditions, but these modiciations were not transmitted to future generations. The gene itself was stable; only the character that the gene controlled varied.
3. Characters that were indistinguishable phenotypically could be the product of different genes.
4. At the same time, each character was the product of many genes. For instance, 50 different genes were known to affect eye color; 15 affected body color, and 10 affected length of wing.
5. Heredity was therefore not some property of the 'organism as a whole,' but of the genes.
6. Genes of the pair did not jump out of one chromosome into another, but changed when the chromosome thread broke as a piece in front of or else behind them. Thus, crossing-over affected linked genes and groups, and was a product of the behavior of the chromosome as an entity.

Digital facsimile from Google Books at this link.