192 cook: evolution through normal diversity 



One is due to the intermingling and interbreeding of two 

 closely related species. It represents a reunion across lots 

 of long separated relatives. Of course, the intergrading forms 

 are hybrids and there is no question regarding the distinctness 

 of the species producing them or the designations of the results 

 of their crossing. The real problem is in recognizing the situation. 



The other situation is that of degradation due to degeneration 

 of certain characters. The irregularity of distribution of the 

 various degrees of such degeneration, and the fact that all de- 

 grees, from apparently perfect individuals to extreme degenera- 

 tion of parts or characters, are sometimes found in one locality, 

 suggest physiological or pathological causes and are individual 

 rather than specific or subspecific in their significance. Such 

 forms are even now recognized as species or subspecies, but with 

 no more justification than there would be in regarding a group 

 of human beings affected with alopecia as constituting a distinct 

 race. "A man's a man for a' that." 



The foregoing hypothetical situations are all represented among 

 fishes today, but are generally not correctly interpreted. Only 

 correct diagnoses of these cases render practicable the uniform 

 application of the previously mentioned definitions of species 

 and subspecies. Then, "true intergradation" designates sub- 

 species. All the other situations are specific. 



EVOLUTION. — Evolution through normal diversity. O. F. Cook, 

 Bureau of Plant Industry. 



Mr. F. L. Lewton, of the United vStates National Museum, 

 has brought to my attention a paper by Thomas Meehan, which 

 contains the following passage : 



The observations on this plant {Impatiens fulva) confirm records 

 I have made during the past quarter of a century that there is an innate 

 power to vary coexistent with the species itself, independent of any 

 conditions of environment. This may be granted without prejudice 

 to the proposition that changes can and do occur at times by the in- 

 fluence of environment, for which there is abundant evidence. It 

 seems proper to present the strong facts on the former side, because 

 of the modem tendency to exalt the latter as the prime motor in evolu- 

 tion.^ 



* Contribution to the life-histories of plants, No. X. Proc. Acad Phila. 1894: 53. 



