CERTAIN TAXONOMIC CHARACTERS OF GRYLLUS. 25 



We have in these various genera instances of creatures differing only 

 in the relative sizes of organs possessed in common by all species of 

 their genus. The dimensions of these organs fluctuate in a way which 

 may be described by the law of error, and they are clearly influenced 

 by environment. Evolution within these genera has not progressed by 

 the addition or subtraction of characters, but by the increase or decrease 

 of common characters. There are, apparently, no centers about which 

 the relative sizes of these organs crystallize, so to speak, except in the 

 case of the wing; and here we have only two centers, or, taking a 

 broad view, three, namely, winglessness (not known to me in Gryllus), 

 short-wingedness, and long-wingedness. The case of the wing-length 

 satisfies all the criteria of a mutation, but it has not given rise to a 

 species, according to the accepted taxonomic usage, any more than 

 have the fluctuations. It could only give rise to two distinct species at 

 the most, in the genus Gryllus, as there are only the two centers about 

 which the wing-length groups itself. Breeding experiments, if they 

 could be carried out, might demonstrate instances of physiological 

 isolation, as in the case of Gryllus domesticus, but such experiments 

 upon all of the described species will always be out of the question. 

 The problems concerning the origin of physiological isolation are import- 

 ant. Equally important and not necessarily connected with these are 

 the problems of the origin of diverse external characteristics. In this 

 and other genera mutation may be a factor in physiological isolation, 

 but the origin of the diversity of many of the important external char- 

 acteristics (hence species?) is indisputably a question of fluctuating 

 variability controlled by environmental conditions. 



7. SUMMARY. 



No specific entities exist in the genus Gryllus which can be demon- 

 strated by any morphological characters thus far studied. "Species" 

 seems to be a human convention of the same sort as "genus." The 

 describing and naming of species here has for its justification conven- 

 ience of reference. 



A large amount of correlation exists between the various taxonomic 

 characters, and this correlation is apparent in the genus as a whole as 

 well as in local samples of it. 



Local environmental influences have an effect upon the taxonomic 

 characters; chiefly, in all probability, upon the length of the ovipositor, 

 the most important taxonomic character. 



Wing-length is markedly dimorphic. Intergrades between the two 

 conditions were not found. Each group fluctuates about its mean to an 

 extent and in a manner similar to the fluctuation of the monomorphic 

 characters. The dimorphism of the wing affects, through correlation, 

 the size of the other organs, especially the tegmina. 



