112 



SCIENCE 



[Vol. LV, No. 1414 



very diiferent matter from that care-free atti- 

 tude of former years wMeli permitted any 

 marked variation not easily interpreted to pass 

 as a mutation. Mutation has become inti- 

 mately a part of that most fundamental and 

 illusive problem of biology, the origin of varia- 

 tion, and mutations apart from the study of 

 their causation are of secondary interest. 



CEnothera material and lines of Drosophila 

 were not the first representatives of impure 

 species to be isolated by the geneticist. The 

 blue Andalusian fowl which cannot be fixed, 

 yellow mice that never have the double dose for 

 yellow, Vilmorin's dwarf wheat which throws 

 tails but fails to produce homozygous dwarfs, 

 single stocks never homozygous for singleness, 

 these and other cases are well known and 

 proven examples of impure species hetero- 

 zygous in their germinal constitution. Certain 

 of them, as the blue Andalusian fowl, throw 

 two homozygous types, in this ease the black 

 and white "wasters." Others produce one 

 viable homozj'gous type. Some impure species 

 rareljr and perhaps never throw homozygous 

 segregates. All agree in this respect that the 

 heterozygote, which breeds true to its jsropor- 

 tion of the progeny, can not be fixed by 

 selective inbreeding although as an impure 

 species it reproduces itself with exactness. 



We have briefiy reviewed conclusions from 

 the intensive study under experimental condi- 

 tions of lines which genetical investigations 

 have established as representatives of impure 

 species. Some of the material is obviously of 

 the sort that would not hold its own under 

 conditions of open competition in Nature, but 

 much of it has been derived from forms not 

 far removed from wild species. There is a 

 broader aspect of the subject of the hybrid 

 deserving of examination, namely, the study of 

 the possibilities of the impure species as a 

 definite component of faunas and fioras. 



rirst of all it is important to bear in mind 

 that if we accept the current theory which 

 places the determination of sex as a function 

 of the reduction or segregation divisions, all 

 unisexual animals are heterozygous for sex 

 factors and for such genes as are responsible 

 for sex-linked characters. For higher animals 

 this means that either the male or female 



carries in single dose a chromosome which is 

 not paired with an equivalent chromosome. 

 For higher plants we should expect the diploid 

 sporophj'te generation to be heterozygous for 

 sex determining chromosomes, a condition for 

 which as yet we have cytologieal evidence from 

 only one type, the liverwort Sphseroearpos 

 studied by Allen and his students, although 

 there is experimental evidence for this condi- 

 tion in other liverworts, in some unisexual 

 mosses, and in certain seed plants, e. g., 

 Melandrium. The behavior of sex-linked char- 

 acters may then be believed to follow an 

 orderly sj'stem in inheritance except as such 

 linkage is broken or as point mutations appear 

 in sex chromosomes. 



But accompanying the sex chromosomes are 

 those groups of chromosomes, the autosomes, 

 responsible for characters not of sex or sex- 

 linked. The unisexual state precludes the 

 possibility of that closest form of inbreeding 

 possible through hermaphroditism and leaves 

 the way open to outbreeding subject only to 

 physiological limitations and to conditions 

 whereby lethals prevent reproduction. That 

 Nature has made extensive use of this encour- 

 agement of outbreeding in various degrees 

 cannot be doubted, and this is best illustrated 

 in man, the most mixed and varied of all ani- 

 mals in the assortment of genes carried by the 

 individual. It is impossible to believe that any 

 human is homozygous for the complex of 

 factors responsible for his individuality. 



Even when, as in most higher plants, the 

 diploid sporophyte generation is bisexual, 

 there have arisen in many lines of evolution 

 conditions that make for very high degrees of 

 genetic impurity. There was a time in the 

 history of botany when workers, following the 

 lead of Darwin, devoted themselves to the study 

 of devices to secure cross-pollination and many 

 and remar]^able are the arrangements described 

 to encourage outbreeding. Volumes have been 

 written on this sidjject and the facts in general 

 are freely admitted. In wind-pollinated forms 

 there is even greater opportunity for promis- 

 cuous pollination unless the shedding of pollen 

 takes place at such a time that stigmas are 

 dusted and the ovules self fertilized before out- 

 side pollen has had an opportunity to reach 



