b ^ BRITISH APHIDES. 



of this undetermined element of time, though it be 

 true that such changes are supposed to be slow in 

 operation. The causes of these changes, indeed, are 

 so obscure that they have been styled by no mean 

 authority " spontaneous " — a term which is doubtless 

 convenient, as it enables us to mask our ignorance. 



For classificatory purposes, however, we must assume 

 that species are permanent until they are proved to be 

 transmutable. The question how long they are perma- 

 nent need not here be discussed. Lingula, Terehratula, 

 and other forms have had a permanence from early geo- 

 logical times, and they are eminently specific ; whilst 

 on the other hand Pterickthys and Gephalasins appa- 

 rently have either died out, or have their hypothetical 

 dilution through other lines of life. 



AYhen external characters of a group only partially 

 suit our purposes of arrangement, we may in 

 many cases obtain material help from an examina- 

 tion of the internal anatomy of its members. 

 Notwithstanding the difi&culties to be expected in 

 following up such inquiries relating to the Aphidida3, 

 Balbiani has made some advance to our knowledge 

 in this direction.* He states that, to a certain extent, 

 there is a constant variation in the number of the 

 ovarian chambers which compose the two great 

 branches of the oviducts in particular genera. Thus, 

 in Siphoiiojjhora the number on each side, he says, is 

 pretty generally from six to seven. In Drepanosiphum 

 it is five, and in Lachnus there appear to be seven 

 loculi. These " gaines de I'ovaire " also would seem 

 to have some numerical relation to the development 

 of the male spermatic capsules. 



These interesting facts, however, will be hardly 

 available for the identification of species ; for the 

 sexual forms of Aphides are in many species very 

 rarely met with, and then, when captured, they would 

 require some manipulative address under the micro- 

 ,scope to make these details available. 



* ' Mem. sur le Generation des Aphides/ i?. 4, Balbiani. 



