CHAPTER 2 



INTRODUCTION TO THE PROBLEM 



The object of this book is nothing more ambitious than to assemble for the first time 

 some material for a prehminary comparison of evolutionary processes, as revealed by 

 cytology, in an ancient and a modern group of plants. The modern group which will 

 be kept particularly in mind, on grounds both of suitability and the accident of close 

 personal acquaintance, is the family of Flowering Plants, the Cruciferae. To this group 

 both the examples discussed in detail in the last chapter belong and a good deal of 

 other cytogenetic information is already available in published form. The ancient 

 group, as will already be obvious, is the assemblage of ferns and fern-hke plants known 

 as the Pteridophyta. For these, so little of the new knowledge has previously been 

 acquired, partly owing to technical difficulty, that the first purpose of the chapters 

 which follow will be to elicit the fundamental data. 



The Crucifers are a family of about 220* genera and 1900* species of Dicotyledons, 

 neither outstandingly primitive nor specially advanced. They are common in Europe 

 but are spread to some extent the world over. Of fossil record they have little or none, 

 but they may reasonably be assumed to have arisen, or at least to have become estab- 

 lished, during the Tertiary period. That they are still actively evolving is suggested 

 first by the prolific development of new forms of domesticated species such as Brassica 

 oleracea, the Cabbage, and secondly by the relatively numerous cases of wild taxonomic 

 species of undoubtedly recent origin which cytogenetic analysis has already detected. 

 Many of the classic 'Jordanian' or ' microspecies ' of Erophila verna first studied in the 

 1850's by Alexis Jordan, and in modern times by Winge (1940), are certainly of this 

 nature, and other examples have already been mentioned in the preceding chapter. 



That hybridization and polyploidy have been potent sources of species formation in 

 the family is known both from observations of the type already quoted for Nasturtium 

 and Biscutella and also from the beautiful experimental work of investigators such as 

 Karpechenko (1928 and earlier), who, in the famous case of Raphanobrassica, induced 

 the formation of closely comparable new forms artificially. That polyploidy itself has been 

 initiated repeatedly throughout the family is known from the simple evidence of com- 

 parative chromosome numbers among related species in many of the genera (cf. 

 Jaretzky, 1932; Manton, 1932 a). t Such simple comparison cannot diagnose the type 

 of polyploidy involved (whether auto- or alio-), nor determine whether the numerical 

 change preceded, followed, or caused the emergence of the species affected by it. 

 It does, however, show beyond dispute that polyploidy in some form or other has 

 entered into the evolutionary history of at least thirty out of the eighty-odd genera 

 examined. This is certainly an underestimate of its frequency, for in several genera 



* Willis's Dictionary, 1925. 



t See also general lists of chromosome numbers by Tischler, Gaiser, Darlington and Ammal, and 



Maude. 



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