Abstract
1. Polymorphic groups of plants are consistently proving to contain species which hybridize readily. 2. A wealth of evidence indicates hybridism to be the cause and polymorphism to be the effect. 3. Betula is a highly polymorphic genus. 4. Polymorphic groups usually show polyploidy. 5. Betula is a polyploid genus containing diploid, triploid, tetraploid, pentaploid, hexaploid, and dysploid species and hybrids. 6. Species of Betula are known to hybridize very readily. 7. The correlation of taxonomic and cytological investigations of XB. jackii and XB. sandbergi presents complete documents of natural hybridization. All of the abnormalities of meiosis which have been considered as characteristic of hybrids have been found to occur in these two species crosses. 8. Hybridization leads to the production of polyploid gametes by the very interesting semiheterotypic division and by non-reduction; consequently heterozygosis is to be considered one of the methods of the origin of polyploidy. 9. Plants, commonly reco...
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