1896.] NATURAL SCIENCES OF PHILADELPHIA. 41 



the style at the base of a narrow tubular corolla, and as it accu- 

 mulates it rises up into the wider part of the corolla where it is ac- 

 cessible to the most short-lipped insects, and where the anthers 

 shelter it from rain." — (Hermann Miiller). In the Blgelovia, Aster 

 and Solidago the tube is seen to be narrow, permitting the rapid 

 rise of the nectar, and probably preventing the insertion of the 

 tongue of large bees. Hence, these flowers are visited only by the 

 smaller species of Perdita, with other small Apidse and Andrenidse. 

 In Verbesina and Helianthns the tube is wider, doubtless permitting 

 the larger bees to readily insert their tongues ; but it it is narrower 

 at the neck than Blgelovia or Solidago, preventing small insects 

 from so readily thrusting their heads inward to stretch for the 

 nectar. The wider tube also may prevent the nectar from rising so 

 far, while in Helianthus there is a large bulb to contain it. 



Solidago canadensis is commonly cultivated in gardens in Europe 

 and there H. Miiller mentions only flies as visiting it (Fertilization 

 of Flowers, p. 321), though he gives a further reference to a paper 

 which 1 have not seen. With us, as has been shown, it is native 

 and visited by several bees. 



THE NATURE OP SPECIFIC DIFFERENCES. 



It is a commonplace observation that specific characters are of 

 all kinds, and may be either strongly marked or diflScult to discern. 

 A very small amount of study teaches us that there is no essential 

 difference between those characters called specific and those called' 

 varietal ; in fact, the very same kind of difference which marks 

 species in one group, may only mark varieties or mutations in 

 another. Thus we come to see that the essential distinctions 

 between species are physiological, the morphological ones being 

 only valid for diagnostic purposes just s© far as they happen te* 

 coincide with the physiological. 



There are even what I have termed " physiological species," i. e.^ 

 species separated only by habit ; not at all, so far as we can judge,. 

 by structure, or if at all, in only a very slight degree. I have else- 

 where cited examples of this kind in Coccidge, but in Hymeoopterai 

 we find many instances in which the tangible characters are reducedl 

 to a minimum. Thus, Schmiedeknecht cites the case of Bomhus: 

 silvarxnn var. $ nigrescens Perez, a submelanic mountain form^ 

 which is only to be separated from B. pratorum by an examination 

 of the genitalia. Among the European Sphecodes also, a study o£ 



4 



