Maech 20, 1903.] 



SCIENCE. 



449 



In these gametes, the unit characters of 

 the plants that bear them are pure. Even 

 in hybrid plants, the pollen grains and the 

 egg cells are not hybrids. According to 

 this hypothesis of gametic purity, there- 

 fore, hybrids follow natural and numerical 

 laws; but these laws are always obscured 

 by new crossing. True intermediate char- 

 acters do not occur. If new characters 

 appear, it is because they have been reces- 

 sive or latent for a generation or because 

 the plant has varied from other causes: 

 they are not the proper results of hybrid- 

 ization. Possibly new characters that ap- 

 pear because of effect of environment or 

 other cause may be impressed on the 

 gamete and thereby be perpetuated. The 

 results of hybridization, then, according to 

 the Mendelian view, are not fundamentally 

 a mere game of chance, but follow a law 

 of regularity of averages; but the results 

 are so often masked that it is sometimes 

 impossible to recognize the law. 



Mendel's law of heredity is recently 

 stated as follows by Bateson and Saunders : 

 'The essential part of the discovery is the 

 evidence that the germ-cells or gametes pro- 

 duced by cross-bred organisms may in re- 

 spect of given characters be of the pure 

 parental types, and consequently incapable 

 of transmitting the opposite character; 

 that when such pure similar gametes of 

 opposite sexes are united together in fer- 

 tilization, the individuals so formed and 

 their posterity are free from all taint of 

 the cross ; that there may be, in short, per- 

 fect or almost perfect discontinuity be- 

 tween these germs in respect of one of each 

 pair of opposite characters.' 



This, in barest epitome, is the teaching 

 of Mendel. This teaching strikes at the 

 root of two or three difScult and vital prob- 

 lems. It presents a new conception of the 

 proximate mechanism of heredity, although 

 it does not present a complete hypothesis 

 of heredity, since it begins with the gametes 



after they are formed, and does not account 

 for the constitution of the gametes nor the 

 way in which the parental characters are 

 impressed upon them. This hypothesis 

 will focus our attention along new lines, 

 and I believe will arouse as much discus- 

 sion as Weismann's hypothesis did; and it 

 is probable that it will have a wider influ- 

 ence. Whether it expresses the actual 

 means of heredity or not, it is yet much too 

 early to say ; but this hypothesis is a greater 

 contribution to science than the so-called 

 'Mendel law' as to the numerical results 

 of hybridization; the hypothesis attempts 

 to explain the 'law.'* 



One great merit of the hypothesis is the 

 fact that its basis is a morphological 

 unit, or at least an appreciable unit, not 

 a mere imaginary concept. This unit 

 should be capable of direct study, at least 

 in some of its phases. It would seem that 

 the Mendelian hypothesis would give a new 

 direction to cytological research, f 



It is yet too early to say how far Men- 

 del's law applies. We shall need to re- 

 study the work that has been done and to 

 do new work along more definite lines. 

 There are relatively few results of experi- 

 ments that can be conformed to Mendel's 

 laAV, because the data are not complete 

 enough or not made from, the proper point 

 of view. We should expect the funda- 

 mental results to be masked when the plants 

 with which we work are themselves un- 

 stable, when cross-fertilization is allowed 

 to take place, or when the pairs of con- 

 trasting characters are very numerous and 

 very complex. Marked numerical resiilts 



• This, I take it, is also the opinion of Bateson, 

 the leading interpreter of Mendel in English; for 

 he calls his new book on the subject (1902) 

 ' Mendel's Principles of Heredity,' as if the 

 heredity idea were greater than the hybridization 

 idea. 



t See, for example, ' A Cytological Basis for the 

 JFendelian Laws,' Bull. Torr. Bot. Cluh, 29, 657 

 (1902), by W. A. Cannon. 



