INTRODUCTION TO THE METHOD 



North America, S. alterniflora, as soon as an accurate chromosome count could be made 

 (cf. Huskins, 1931) of the three forms in question. The endemic species S. Townsendii 

 could then be seen to contain twice the sum of the gametic chromosome numbers of 

 the two putative parent species and was therefore clearly the hybrid between those 

 species, which had doubled its chromosomes and become fertile and true breeding in 

 consequence. 



Comparative morpholog}' of chromosomes may also give higlily significant information, 

 some very striking examples being the work of Babcock (1947) and his colleagues and 

 collaborators on Crepis in plants and the numerous studies on various genera of flies, 

 notably Drosophila and Sciara, some of which have been summarized recently by 

 Dobzhansky (1937) and by White (1945). From these it is clear how important a part 

 gross structural changes have played in the evolution of species in these groups. 



A still more powerful tool lies in the accurate analysis of chromosome pairing, either 

 at meiosis or, in the special case of the Diptera, in the salivary glands. The detection 

 of failure of pairing or irregular pairing in a wild plant or animal will often reveal a 

 hybrid with certainty where other evidence is inconclusive or misleading. The classic 

 cases of Drosera obovata (Rosenberg, 1909), Rosa (Blackburn and Harrison, 192 1), 

 Hieracium (Rosenberg, 191 7), etc., are examples which could be indefinitely multiplied. 

 This type of inquiry has been of enormous benefit to pure taxonomy wherever natural 

 barriers between species have been obscured by the existence of unrecognized hybrid 

 forms, some of which, as in the case of Rosa, may have been perpetuated from remote 

 ages by the adoption of a non-sexual mode of reproduction. The presence of extra 

 chromosomes is revealed by the formation of multivalent groups. These may occur 

 singly as in the sex chromosomes oiRumex acetosa, in which the male plant has a trivalent 

 group in place of the bivalent pair of the female, in this case betokening perhaps no 

 more than the fragmentation of one sex chromosome. The presence of multivalent 

 groups on a considerable scale more commonly denotes the presence of duplicated sets 

 of chromosomes which constitutes the phenomenon of polyploidy.* This may be 

 autopolyploid, i.e. due to exact duplication of identical gametic sets with at most only a 

 minor amount of mutational difference of ' genie ' origin to distinguish them. Tetra- 

 ploid Biscutella laevigata (see below) in the Swiss and Austrian Alps is an example of this 



* Polyploidy (Winkler, 1916) is the name given to the state of a cell, organism, or tissue, in which 

 the number of chromosomes contained in the nucleus is a simple multiple of the chromosome number 

 of some other, related, cell, organism or tissue. When the nuclear cycle of the higher plants was fiist 

 worked out at the beginning of the century the words haploid and diploid were introduced to designate 

 the reduced and unreduced nuclear states of the same organism. When it was later realized that ether 

 multiples of the basic gametic number could exist, the term haploid has in certain contexts given place 

 to monoploid, and additional terms such as triploid, tetraploid, pentaploid, hexaploid and so on have 

 had to be introduced to describe the various members of a series, which collectively represents the state 

 of polyploidy. The word heteroploid is sometimes used as a synonym for polyploid {e.g. by Sharp, 1934), 

 as also is euploid (Tackholm, 1922). Special types of polyploidy, namely, autopolyploidy and allopoly- 

 ploidy (Kihara and Ono, 1926), are explained in the text on pp. 8-9; these have recently been abbrevi- 

 ated to autoploidy and alloploidy (Clausen, Keck and Hiesey, 1945). 



The opposite condition in which the nuclei of related cells differ by some quantity which is not that 

 of a simple multiple of a basic or monoploid set of chromosomes is designated aneuploidy (Tackholm, 

 1922) or dysploidy (see Clausen. Keck and Hiesey, 1945). 



