Species and Species Change 119 



cannot yet be diagnosed on the basis of conventional taxonomic 

 characters but differ in chromosomal arrangements (Rothfels, 

 1956). These different chromosomal arrangements may be seen 

 readily in specially collected and prepared salivary gland chromo- 

 somes of the larvae (Duncan, 1959; Basrur, 1959). 



In a few instances, no known reliable distinguishing features of 

 a visible nature have been detected between related species. In 

 field crickets of the genus Acheta of eastern North America, five 

 sympatric native species differ to the eye only in average char- 

 acters of color and proportions (Fig. 46), but in each species the 

 males have a distinctive mating song (Fulton, 1952; R. D. 

 Alexander, 1957). Breeding colonies set up on the basis of these 

 songs behave reproductively as any good "morphological" species. 



It is therefore possible for genetically distinct species to differ 

 physiologically or biochemically but to lack traits affording visible 

 identification. Ever since this situation was appreciated, alarm has 

 been expressed that perhaps our identifiable species are in truth 

 conglomerates of undetected but genetically distinct species. How- 

 ever, in the case of several previously unidentifiable species known 

 to be distinct by genetic tests, intensive search has unearthed either 

 previously unobserved differences or a distinctive modal distribu- 

 tion of characters. In insects of the genus Drosophila, for example, 

 the species pair pseudoobscura and persimilis were long considered 

 inseparable except by genetic tests, as were five species in the D. 

 tvillistoni group. However, Spassky (1957) found reliable identifica- 

 tion characters in several parts of the male genitalic structures in 

 these species of Drosophila. In plants, two genetically distinct 

 species of UvuJaria, grandifiora and pcrfoliata, were long considered 

 indistinguishable on any reliable basis; Anderson and ^^^litaker 

 (1934) showed that individuals of the two species could be sorted 

 accurately by using statistical indexes involving combinations of 

 several characters. Several investigators have found that various 

 biochemical assays such as paper chromatography and electro- 

 phoresis (Cassidy, 1957; Sibley, 1960) have value in separating 

 species otherwise difficult to identify (Micks, 1954; Lewallen, 

 1957). It seems that eventually diagnostic characters will be found 

 between all species. 



Biological and Ecological Characteristics 



A detailed study of two or more species always discloses some 

 biological difference between them. In many cases the difference 



