Kendall: what characters distinguish species? 189 



Ichthyologically, I, myself, discern two kinds of phantoms, 

 which I designate as species. One is a taxonomic species, 

 the other a natural species. At times they may blend into one 

 but not always. Both conform to the definition given by my 

 leaders and both imply development of one form from another, 

 but there are two lines of development or derivation to be con- 

 sidered which I will designate as horizontal and vertical. The 

 first refers to derivation, development, and relationships on 

 the surface plane of a given time, past or present. The second, 

 to derivation and development in time from the past to the present. 

 Any cross-section of the vertical will present a horizontal plane 

 of development. In taxonomic considerations of species and 

 their subdivisions of living fishes, the tendency appears to be 

 to regard them in their horizontal aspect. That is, in their 

 relations to each other in the period of time in which they exist. 

 If the vertical is considered at all it is usually as though the 

 organism of the past was the same as the present. For instance, 

 it is stated that Salmo salar sehago is derived from Salmo salar, 

 as though the parent stock was the same as the Atlantic salmon 

 of today. It may or may not have been. That is a point to 

 be considered vertically. My meaning may become clearer 

 if we imagine a longitudinal section through the vertical from 

 the past to the present time. 



During some period of time, recent or remote, through limita- 

 tions to interbreeding and other causes, it is conceivable that 

 two horizontal extremes developed differences of character 

 which graded in toward the center. The persistence to the 

 present time of these interbreeding connections constitute 

 a perfect intergradation geographically between the two extremes 

 of the present time. Again, it is conceivable that by expansion 

 geographically of one or the other or both of the differentiating 

 extremes, there is an actual or relative contraction of the in- 

 termediate connections. In other words, there is a tendency 

 toward segregation of the extremes through weakening of the 

 interbreeding connections. 



Both phylogenetically and taxonomically in this situation 

 at the present time there can be only one species, not withstand- 



