KENDALL: WHAT CHARACTERS DISTINGUISH SPECIES? I9I 



development, progressing without a differentiation at the margin, 

 should be segregated — -put off at a way station, so to speak — 

 and, by virtue of little or no change of environment, should 

 be retarded or arrested in its phylogenetic development. 

 Then suppose that, at a later period, similar segregation takes 

 place, which for like reasons is also retarded or arrested. Both 

 of these forms, persisting in their isolation and limited inter- 

 breeding to the present day, to the horizontal observer would 

 appear as two intermediate forms, that is to say, connecting 

 links between the respective species resulting from each of the 

 main lines of phylogenetic development, previously referred 

 to. The result of the first mentioned segregation would resemble 

 the species at the present end of the main line other than the 

 one resulting from the line in which the segregation originated, 

 more nearly than would the result of the later segregation. 

 So, although they appear as intermediate forms between ex- 

 isting species, they are not. They represent, rather, vertical 

 intermediates between the common ancestral stock and the 

 one species at the present end of the line from which they them- 

 selves were segregated. They form no intergradation, continuous 

 nor interrupted, between the living species, but between an 

 ancestral species different from the present species, and are, 

 therefore, living fossils as it were. They may have, and probably 

 did pursue, some small degree of development themselves. They 

 should, I believe, be regarded as distinct species. 



In my hypothesis I have designated only two of this latter 

 character. There might be many. The more there are, the 

 more difficult would it be to recognize the situation. But when 

 the intergradation is interrupted, as it is in these instances, 

 and particularly where each form is somewhat isolated at the 

 present time, as in such instances it is likely to be, I believe 

 that specific definition of my leaders fit the cases. The inter- 

 ruptions are represented by certain characters, which, whether 

 little or great, entitle the forms to specific designation. They 

 are natural species and should be thus taxonomically recognized. 



There occur to me two other situations that might be, in fact 

 have been, mistaken for real intergradations. 



