INTRODUCTION. 



17 



Erica, Rhododendron, Achimenes, Calceolaria, Gladiolus, 

 and Hippeastrum) gardeners have crossed the species 

 intentionally and unintentionally in the greatest variety 

 of ways, and from the forms obtained they have used 

 those most desirable for further cultivation. The off- 

 spring of these complicated hybridization products are 

 naturally almost always very varied. On the other hand, 

 there are exceptions to this rule. Sweet particularly 

 emphasizes the fact that the same hybrid form is obtained 

 from the crossings of several complex 1'i'lnri/oniiiin hy- 

 brids. Such constant complex Pelargonium hybrids are, 

 according to him, P. involucratum X P- ignescens, and 

 P. iiioslyna; X P. ignescens. It has already been men- 

 tioned that Erica and several Salix hybrids on crossing 

 furnish offspring of constant form. 



Cross-breeds and Hybrids. According to a dictum 

 hybrids of two different varieties of one species are desig- 

 nated as cross-breeds, and hybrids of two different species 

 as hybrids. As the term varieties is vague it is necessary 

 at this point to remember that only varieties which 

 breed true, as well as races, or subspecies, can with cer- 

 tainty transmit in some degree their properties. Un- 

 stable breeds which are designated varieties are useless 

 in the study of hybridization. 



Many writers have taken great pains to discover a 

 sharp distinction between cross-breeds and hybrids. They 

 hold to the expectation that by researches in hybridiza- 

 tion a border line between species and subspecies will be 

 fixed. Giirtner, who in many places in his works has 

 declared that the conditions of the hybrids demonstrate 

 clearly the specific differences or similarities of the 

 parent-forms, would soon retract if he attempted to de- 

 velop any connection or continuity by the literature of 

 variety hybrids. Herbert and Naudin have through 

 many researches arrived at the conviction that it is im- 

 possible to draw a sharp borderline between crosses and 

 hybrids; nevertheless, later botanists have always sought 

 such a fixed difference. 



The following propositions have been formulated : 



1. The pollen of a cross-breed is normal; there are 

 more or less numerous deformed pollen grains in a 

 hybrid. 



2. The fertility of a cross-breed is normal; that of a 

 hybrid is distinctly subnormal. 



3. Hybrids of two species having differently colored 

 flowers bear flowers of modified coloring. Plants with 

 irregularly dappled flowers are produced from the cross- 

 ing of varieties. They behave similarly in regard to 

 coloring, marking, and formation of fruit, and other 

 properties. 



4. Cross-breeds have a decided inclination in later 

 generations to revert entirely to the parent forms. 



These four propositions are in general correct, but 

 give very little help to a final decision in doubtful cases. 

 The hybrids of the red and blue Anagallis arvensis must 

 according to the pollen he considered a hybrid, but 

 according to the production of bicolored flowers, a cross- 

 breed. Datura hybrids, which are manifestly character- 

 istic hybrids in other ways, readily revert completely to 

 the parent species. Hybrids whose fertility is apparently 

 in no way weakened have already been specified. The 

 rule can, therefore, be set forth that hybrids of very 

 nearly related races usually show the properties attrib- 



uted to cross-breeds, but it is another matter to establish 

 a sharp boundary line between race-cross-breeds and 

 species-hybrids. 



Several other properties of cross-breeds have been 

 added by which they may be distinguished from species- 

 hybrids. Gartner has maintained that cross-breeds of 

 a similar origin will be very unlike one another even in 

 the first generation, while hybrids of the first generation 

 will be of the same form. This assertion, which has been 

 repeated by others, is entirely unjustified. The multi- 

 plicity of forms of the species-hybrids of Abutilon, Passi- 

 flora, Hieracium, and so forth has already been pointed 

 out and, on the other hand, race-cross-breeds of the first 

 generation are usually as similarly formed as true hy- 

 brids. Again, it is often maintained that the varieties 

 of one and the same species if crossed with another species 

 produce the same hybrid forms. Giirtner especially has 

 emphasized this alleged behavior of " varieties," although 

 he must have known that Kb'lreuter had already noted 

 the transmission of flower-coloring in races of Mirdbilis, 

 Dianthus, and Vcrbascum, the flower-filling (Bliithen- 

 fullung) of Aquilegia and Dianthus, and the form and 

 leaf-shape of races of Nicotiana tabacum and Hibiscus. 

 The white-blooming Datura ferox and D. strammonium 

 typ. (a white-flowered form) with the smooth-fruited 

 race (var. tertolonii) of the same species forms a blue- 

 flowered hybrid. Nympha-a Mm X N. rubra is different 

 from N. lotus X N. dentata. It is unquestionable that 

 properties of races and so-called varieties which are 

 hereditary in pure-breeding are also transmitted to their 

 hybrid offspring. It is self-evident that forms whose 

 normal offspring behave in an unstable fashion will also 

 produce polymorphous hybrids and that the unstable 

 characteristics of varieties will entirely disappear in the 

 products of the hybridization of pure species. 



The facts in short are as follows : The nearer the 

 morphological and systematic relationships of the parent 

 forms the less does the procreative power of the hybrid 

 depart from the normal. The farther the parent forms 

 are from one another the more commonly is the fertility 

 of the hybrid weakened. Exceptions, however, are not 

 infrequent. 



The nearer the parent forms are related to one an- 

 other, the more frequently does the offspring of hybrids 

 show reversion to the parent forms. 



Hybrids of nearly related parent-forms show in their 

 fruits the characteristic properties of the parents un- 

 blended a.nd side by side, but in hybrids of very different 

 parent forms this is seldom seen. 



The most asymmetrically variegated flowers (Mira- 

 bilis, Camellia, Mimulus, Petunia and so forth) have, 

 moreover, originated from the offspring of hybrids. 



The propositions of Focke, although published in 

 1881, are not subject to modifications in principles 

 even at the present time. Much literature on the sub- 

 ject of the sterility of hybrids might be quoted and 

 some references might be made to extensions and addi- 

 tions of a more or less important character to the data 

 and propositions set forth, but this seems needless for 

 the purposes of this chapter and this research. 



