CHAPTER XVII 

 CYTOLOGY AND TAXONOMY 



In recent years, cytology has found a new field of usefulness as an ally 

 of taxonomy. As every student of biology is aware, the task of taxonomy 

 is that of arranging animals and plants into a hierarchy of systematic 

 units — species, genera, families, orders, etc. — according to the degree 

 of their relationship. In earlier days "relationship" meant merely 

 resemblance in externally visible characters or nearness of approach to 

 certain ideal standards. Modern taxonomy is much more than this. 

 It strives to improve sj^stems of groupings made for the sake of con- 

 venience, but it also seeks the natural causes of the likenesses and 

 differences in character observed, and it does this knowing that true 

 relationship has its basis in community of origin. 



Modern taxonomy differs from that of earlier centuries in two other 

 important respects: it makes use of a greater diversity of evidence, and 

 like other branches of biology it has supplemented observation with 

 experiment. In its effort to discover true relationships and origins, it 

 not only makes use of the usual morphological characters, but is quick 

 to seize upon evidence afforded b}^ physiological behavior, ecological 

 relations, geographical distribution, serological interactions, cytological 

 characters, genetical behavior in controlled crosses, and, in some measure, 

 the fossil record. In short, it is now more conscious of its integral share 

 in accounting for the distribution of organic types in space and time. 



Tliis chapter affords a glimpse of cytotaxonomy , in which cytological 

 characters, chiefly the number, morpholog}^, and behavior of chromo- 

 somes, are employed in the task of determining true natural relationships. 

 The immediate aim of this new alliance of cytologj^ and taxonomy is to 

 establish and measure correlations between such cytological characters 

 and the natural taxonomic units founded on field observation and con- 

 trolled experiment. Its further aim is to evaluate alterations in chromo- 

 some complements as factors in the development and diversification of 

 these units, and so to gain a more broadly based conception of the origin 

 of the diversity observed in the living world. 



This type of study has been carried on more extensively with plants 

 than with animals. This is due in large measure to the fact that poly- 

 ploidy occurs very widely in plants, especially among angiosperms, 

 whereas among animals it is comparativelj^ rare. In both kingdoms, 

 however, significant results are being achieved. 



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