Makch 20, 1903.] 



SCIENCE. 



453 



that are not Mendelian. Dearies denomi- 

 nated the law of numerical segregation 

 as the 'law of separation of characters in 

 crosses.' Like Mendel, he had found that 

 merely to cross 'varieties' or 'species' is of 

 no avail in the study of fundamental prob- 

 lems; for the varieties and species that we 

 know are mere systematic groups with char- 

 acters of all kinds and' degrees. We must 

 cross characters or units, not species. 



Now, every unit character he conceives 

 to be represented in the germ by a 

 pangene. This pangene may be active, 

 in which case the character appears in the 

 plant ; or it may be dormant, in which case 

 the character is not visible, or for the time 

 being is lost. Active pangenes may at any 

 time become latent, or latent ones may be- 

 come active. 



Mendel's law results from an interchange 

 of contrasting characters. True physiolog- 

 ical or elementary species differ from each 

 other by new unit characters. They have 

 arisen by progressive mutation. The char- 

 acters are not contrasting or differentiat- 

 ing. One species has one kind of pangene, 

 another species another kind of pangene. 

 On combining these there can be no inter- 

 change of characters, and therefore no 

 Mendelism. There is nothing for one char- 

 acter to exchange against the other. In 

 the case of true progressive mutations, 

 therefore, upon which the progress of the 

 plant race depends, there can be no Men- 

 delizing. Hybrids of these cases are in- 

 termediates, or else follow only one or the 

 other of the parents. 



Now, varieties differ from true mutative 

 species in the fact that they have con- 

 trasting characters. ' These characters are 

 represented by their special kinds of 

 pangenes. The pangene may be active or 

 passive. That is, the variety may be a 

 variety because one or more of its char- 

 acters has become latent (retrogressive) or 



because characters have become active 

 (degressive). When these characters are 

 crossed, there is an interchange of the 

 pairs. Both parents bear the same unit 

 character, but this character is active in 

 the one and dormant in the other. The 

 hybrid receives an active pangene from one 

 parent and a similar but inactive pangene 

 from the other. When these two units 

 unite, the calculus of chance determines 

 that there shall reappear in the second gen- 

 eration equal numbers of both the parental 

 units, and half of the whole that are still 

 hybrids and break up in the same ratio in 

 the third generation. That is, true Men- 

 delism is confined to crossings of retro- 

 gressive and degressive varietal characters. 



There are, therefore, two general classes 

 of hybrid formation— the isogons, giving 

 rise to crosses in which two antagonistic 

 parental characters reappear in numerical 

 order (Mendelian cases) ; anisogons, giving 

 rise to crosses in which two antagonistic 

 sometimes separate imequally, but ordi- 

 narily do not separate at all. When only 

 one parent is represented in the offspring, 

 we have the 'imisexual crosses' of Macfar- 

 lane or the 'false crosses' of Millardet. 

 These are cases in which there are no true 

 contrasting characters. Spillman has re- 

 cently explained the false hybrids by sup- 

 posing that the plants in this case are self- 

 fertile and sterile with other pollen. That 

 is, A is fertile with A, B with B, but A is 

 not fertile with B nor B with A; there 

 results, therefore, no true crossing. This 

 hypothesis should be capable of experi- 

 mental proof or disproof. 



The isogon hybrids are of all degrees 

 of complexity, and classification of them 

 will at once show how far we have already 

 got away from the pld systematic idea of 

 variety-hybrids and species-hybrids. Hy- 

 brids between plants that differ only in 

 one unit-character are monohybrids. These 



