546 PROCEEDINGS OF THE AMERICAN ACADEMY. 



variation may possibly prove conformable with Mendelian principles. 

 Take, for example, the height of peas. It has been found in certain 

 crosses of a tall with a dwarf variety of pea, that the hybrid has an 

 intermediate height. Now, if the hybrid produces pure germ-cells, dwarf 

 and tall respectively, in equal numbers, the next generation will consist 

 of three classes of individuals, dwarf, intermediate, and tall, in the pro- 

 portions, 1:2:1. But if each of the original characters should undergo 

 disintegration, we might get a dozen classes, instead of three, resulting 

 in a practically continuous frequency-of-error curve. 



Summary. 



1. The basic principle in Mendel's discoveries is that of the purity of 

 the germ-cells ; in accordance with this a cross-bred animal or plant pro- 

 duces germ-cells bearing only one of each pair of characters in which its 

 parents differ. From it follow the occurrence in the second and later 

 hybrid generations of a definite number of forms in definite numerical 

 proportions. 



2. Mendel's principle of dominance is realized in the heredity of a 

 considerable number of characters among both animals and plants. In 

 accordance with this principle, hybrid offspring have visibly the character 

 of only one parent or the other, though they transmit those of both parents. 



3. In other cases the hybrid has a distinctive character of its own. 

 This may approximate more or less closely the character of one parent 

 or the other, or it may be entirely different from both. Frequently the 

 distinctive hybrid character resembles a lost ancestral character. In 

 some cases of this sort, as in coat-color of mammal's, the hybrid character 

 probably results from a recombination of the characters seen in one or 

 both parents, with certain other characters latent (that is, recessive) in 

 one parent or the other. 



4. There have been observed the following exceptions to the principle 

 of dominance, or to the principle of purity of the germ-cells, or to both : — 



(a) Mosaic inheritance, in which a pair of characters ordinarily related 

 as dominant and recessive occur in a balanced relationship, side by side 

 in the hybrid individual and, frequently, but not always, in its germ-cells 

 also. This balanced condition, once obtained, is usually stable under 

 close breeding, but is readily disturbed by cross-breeding, giving place 

 then to the normal dominance. 



(b) Stable (self-perpetuating) hybrid forms result from certain crosses. 

 These constitute an exception to both the law of dominance and to that 



