116 



Species and Species Change 



f 



Two daughter 

 phyloqenetic lines / 

 progressing ^ 

 through lime 



Division of ^---^ f 

 phylogenetic line 



Parental 

 phylogenetic line 

 progressing 

 through tinfie 



■Species 5 ond 6 

 A 



o a> », 





Species \ 

 (Cross-section of 

 phylogenetic line at 

 this level in time) 



Fig. 45. Diagram of hypothetical phylogenetic hne to illustrate the relation- 

 ship between phylogenetic lines and species. 



after generation; individuals cross freely with others of their own 

 kind but do not interbreed with individuals of other kinds. This 

 repetitive and essentially genetic concept of a species is old, without 

 doubt dating back at least to the prehistoric time when man began 

 keeping flocks and sowing seeds. In spite of the antiquity of the 

 species concept, confusion still exists as to what is a species and 

 how the term should be used properly. Much of the confusion stems 

 from the fact that species are for the most part dynamic, not static, 

 and it is frequently difficult to find readily understood terms which 

 will define species in respect to their changing nature. 



Genetic isolation has been stressed above as a criterion of species. 

 In the processes leading to an increase in the number of species, 

 however, daughter species of certain types may pass through stages 



