STATEMENT OF PROBLEM. 3 



rarely when species are crossed." De Vries (1905, pp. 253, 280) concludes 

 that blending and particulate inheritance of qualities characterize the 

 offspring of crossed species, whereas an alternative inheritance of qualities 

 is characteristic of the offspring of a species crossed with a variety* or of 

 two varieties crossed iiitfr se. 



In the case of alternative inheritance there often is exhibited an extremely 

 suggestive phenomenon. When hybrids showing such inheritance are 

 crossed inter sc there is a segregation of the various alternative character- 

 istics into different individual offspring. This is the discovery of Mendel 

 (1866). t The attempt has naturally been made to generalize Mendel's 

 law — to make it apply universally. In my own study the applicability of 

 this law has been kept constantly in mind. 



* It is to be recalled that in the De Vries system a variety differs from its parent 

 species either in that a characteristic of the species has become latent in the variety or 

 in that a characteristic which was latent in the species has reappeared in the variety. A 

 new species, on the other hand, differs from its parent species in the acquisition of one 

 or more wholly new characteristics. "In normal fertilization and in the intercrossing 

 of varieties all characters are paired." Hence the paired characters struggle together in 

 the zygote and the stronger one of the pair dominates or covers over the weaker one. 

 Thus inheritance is alternative or exclusive. "In crosses between elementary species 

 the differentiating marks are not mated." Hence there is no such struggle between 

 characteristics ; consequently those of both parents reappear in the offspring, interdigi- 

 tating. 



tThe rediscovery of Mendel's work simultaneously by De Vries and by Correus in 

 1900 will always rank as one of the interesting coincidences in the history of science. 

 There is evidence that others had independently discovered this law in their own work 

 in the last third of the nineteenth century, but the history of this law is still to be written. 

 I may note that Haacke, in 1S93, as a result of extensive breeding of animals, expresses 

 the law of purity of the germ cells. He has the theoretical idea that inheritance is con- 

 veyed both by the plasma (P) and the nucleus (Kern, K). In the union of dissimilar 

 races two kinds of plasma (P and P') and two kinds of nuclear material (K and K') may 

 be distinguished. On page 236 he says : 



Die beideu verschiedenen Plasmen P und P die sicli bei der Befruchtung vereiuigt haben, trennen 

 sich wieder bei der Reductionsteilung der Keimzelle, und dasselbe gilt von den beiden Kemstoffe K 

 und K'. Diese Treanimg ist in niinchen Fallen, wie es scheint, eioe vollige, so dass die Plasmen lind 

 die Kerustoffe. ab^eselieii von deu mehr oder minder weitgelienden. aber niemals voUkom-iienen 

 Ausgleichun^en ilirer Eigenschaflen, die durch gegenseitige Beeinflussung stattfinden miissen, ebenso 

 rein ans der Vereiniguug hervorgehen, als sie in diese hineingetreten sind. 



Still further Haacke recognizes that in the separation of qualities that occur in the 

 reduction period of the hybrid germ cells, those from different parents may gather into 

 one germ cell. Since this occurs in accordance with the laws of chance (worked out in 

 an example by Haacke), we have various combinations of characteristics in the second 

 hybrid generation. Because of the purity of the germ cells it will often happen that 

 mice having certain qualities will, when bred together, produce only those qualities, 

 however complex their ancestry. For example, white dancing-mice bred together will 

 produce nothing but white dancing-mice. Haacke's results seem to have been overlooked 

 by recent experimenters. 



