STUDIES OF VARIATION IN INSECTS 319 



beetles of the family Carabidas (to which family Pterostichus 

 belongs). In the family Dytiscidas (predaceous water-beetles) 

 the elytral striae are also used as classificatory characters. A 

 curious female dimorphism exists in some Dytiscid species in 

 which one form of female has the elytra deeply grooved, while 

 the other form has smooth elytra. The significance of this 

 dimorphism is not known. 



GENERAL RESULTS AND SIGNIFICANCE. 



Blastogenic and Acquired Variations. — The importance of 

 distinguishing, in any study of variation considered as a factor 

 in species-forming, between those variations in the animals 

 under observation which are truly blastogenic and those which 

 are, in part at least, acquired by reaction to some causative 

 influence from without during the immature life (development) 

 of the individuals is obvious. If acquired characters are non- 

 heritable, then, while the rigor of selection among adults may 

 and will take into account any variations, either blastogenic or 

 acquired which may exist in these adults, the selected survivors 

 will, in fact, tend to transmit, and thus retain in the species, 

 only those variations which are blastogenic. If acquired char- 

 acters are heritable then the importance, perhaps, although not 

 the interest of such a distinction between the two categories of 

 variation may be lessened. But undoubtedly a majority of 

 working naturalists believe that the inheritance of acquired 

 characters is yet unproved. 



With the importance of this distinction well in mind — indeed 

 with the belief that variation study without this distinction in 

 mind has not much claim to attention from biologists intent on 

 discerning the factors in a method of evolution — we have tried 

 to point out in this paper in the case of each insect or charac- 

 teristic studied, the character, blastogenic or acquired, which 

 the variations discussed possess. For example, the variations 

 in the pattern of Diabrotica,^ Hippodamia and Vespa (insects 

 of complete matamorphosis with all adult external structures 



• By reference to the table of contents the position in this paper of the par- 

 ticular discussion of any variation referred to in this general part of the paper 

 mav be found. 



