GENETIC BASIS 21 



There are not at the present time enough experhnental 

 data even for a rough estimate of the possible frequency of 

 interspecific and intergeneric crosses in different groups of 

 organisms. For various reasons it has been simpler to at- 

 tempt species and generic crossing on a large scale among 

 the higher plants than among the insects or the vertebrates. 

 The number of wide crosses known among the higher plants 

 might equally well be due to a wdder tolerance of such 

 miscegenation there, or to the much lesser number of artificial 

 crosses that have been attempted among the vertebrates and 

 insects, for all w^e know at the present time. The fact that 

 species hybrids and semifertile generic hybrids have been 

 so frequently obtained among the fishes looks suggestive but 

 can scarcely be taken as conclusive. Aside from the higher 

 plants, the one group of organisms the largest numbers of 

 which have been successfully raised in capti\dty is the fishes, 

 and it is among them that the largest number of vertebrate 

 crosses permitting gene exchange between distinct genera has 

 been reported. 



For the higher plants the actual experimental evidence is 

 more extensive than many biologists realize. From the time 

 when Camerarius first announced that the higher plants were 

 sexual in nature, until the early days of genetics, a whole 

 series of investigators pursued the subject, first estabhshing 

 in the face of stiff opposition (Zirkle, 1935) the fact that such 

 hybrids could really be made, and then launching an attempt 

 to summarize and analyze the results of these crosses. The 

 total number of precise scientific controlled experiments in 

 this era was staggering. Von Gartner, the outstanding of 

 these hybridizers, worked with around 700 species. He at- 

 tempted more than 10,000 controlled, recorded crosses and 

 produced 250 different hybrids. 



When the possibihties of hybridization became apparent, 

 it was carried on extensively by amateurs and horticulturists 

 for practical purposes. This work still continues. While 

 some scientists were still debating whether intergeneric 

 crosses such as those made by Janaki-Anamal were a sci- 



