September -io, lilO.!.] 



SCIENCE. 



405 



delian principles. Take, for example, the 

 liei.^ht of peas. It has been foiind in cer- 

 tain crosses of a tall with a dwarf variety 

 of pea, that the hybrid has an inter- 

 mediate height. Now, if the hybrid pro- 

 duces pure arerm-eells, dwarf and tall re- 

 spectively, in equal numbers, the next gen- 

 eration will consist of three classes of 

 individuals, dwarf, intermediate and tall, 

 in the proportions 1:2:1. But if each of 

 the original characters should undergo dis- 

 integration, we might get a dozen classes, 

 instead of three, resulting in a practicallj- 

 continuous frequeney-of-error curve. 



SUMMARY. 



1. The basic principle in Mendel's dis- 

 coveries is that of the purity of the germ- 

 cells; in accordance with this a cross-bred 

 animal or plant produces germ-eells bear- 

 ing onlij one of each pair of characters in 

 which its parents differ. From it follows 

 the occurrence in the second and later hy- 

 bi'id 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 prin- 

 ciple, hybrid offspring have visibly the 

 character of onlj- one parent or the other, 

 though they transmit those of both parents. 



3. In other eases the hybrid has a dis- 

 tinctive character of its own. This may ap- 

 proximate more or less closely the char- 

 acter of one parent or the other, or it may 

 be entirely different from both. Fre- 

 quently the distinctive hybrid character 

 resembles a lost ancesti-al character. In 

 some cases of this sort, as in coat-color of 

 mammals, the hybrid character probably 

 results from a recombination of the char- 

 acters 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 follow- 

 ing exceptions to the principle of domin- 

 ance, or to the principle of puritj' of the 

 germ-cells, or to both: 



(a) Mosaic inheritance, in which a pair 

 of characters ordinarily related as domin- 

 ant and recessive occur in a balanced re- 

 lationship, side by side in the hybrid indi- 

 vidual and frequently in its germ-cells 

 also. This balanced condition, once ob- 

 tained, is usually stable under close breed- 

 ing, but is readily disturbed by cross-breed- 

 ing, giving place then to the normal 

 dominance. 



(6) Stable (self-perpetuating) hybrid 

 forms result from certain crosses. These 

 constitute an exception to both the law of 

 dominance and to that of purity of the 

 germ-cells. For the hybrid is like neither 

 parent, but the characters of both parents 

 exist in a stable union in the mature germ- 

 cells produced by the hybrid. 



(c) Coupling, i. e., complete correla- 

 tion may exist between two or more char- 

 acters, so that they form a compound unit 

 not separable in heredity, at least in certain 

 crosses. 



(d) Disintegration of characters ap- 

 parently simple may take place in conse- 

 (juence of cross-breeding. 



( e ) Departures from the expected ratios 

 of dominants to reeessives may be ex- 

 plained in some cases as due to inferior 

 vigor, and so greater mortality, on the part 

 of dominants or reeessives respectively. 



(/) Cases of apparent reversal of dom- 

 inance may arise from 'false hybridization' 

 (induced parthenogenesis). Possibly in 

 other eases the determination of dom- 

 inance rests with circumstances as yet 

 unknown. 



5. Mendel's principles strengthen the 

 view that species arise by discontinuous 

 variation. They explain why new types 

 are especially variable, how one variation 



