SPECIAL TECHNIQUES 99 



servation. In a genus like Fraxinus, in which species are 

 separated for the most part by vague and inconstant dif- 

 ferences in texture, pubescence, etc., extrapolation will be 

 difficult, though not impossible. The more closely related 

 the entities involved and the more similar they are morpho- 

 logically, the more difficult will it be to find differences that 

 lend themselves to precise description and measurement. In 

 the higher plants, however, with persistence, it has always 

 proved possible to find suitable characters. It must be ad- 

 mitted that the techniques of putting such differences as leaf 

 shape, leaf texture, and branching patterns into measurable 

 form are still in the exploratory stage, but several that have 

 been worked out for particular cases seem to be rather gen- 

 erally apphcable. How far these methods can be used with 

 other kinds of organisms it would be difficult to say. Because 

 of the relatively simple nature of their development, plants 

 exhibit their species differences in less complicated ways than 

 does, for example, an insect wing or a vertebrate tooth. 



In trying out such a method as that described above, one 

 elementary fact is of great importance. If possible the work 

 should be done in the field, at least in a preliminary way. By 

 taking squared paper to the field it will often be possible to 

 measure at least a few of the more obvious differences in a 

 population and make a preliminary determination of what 

 characters are tending to cohere in that population. As the 

 cohering center is apprehended more and more closely, the 

 sets of characters that go together will be more and more 

 clearly seen. One will thus be able to collect those specimens 

 and to concentrate on the study of those characters that are 

 the most effective. 



In interpreting and measuring the results of interspecific 

 introgression, one of the most difficult and challenging prob- 

 lems is the effect of a few genes from one species when in- 

 troduced into the genetic background of the other. The 

 greater the morphological hiatus between the two hybrid- 

 izing entities, the more difficult does it become to predict 

 the impact of such a recombination or to interpret it after it 



