492 Studies in the Theory of Descent. 



That the form-divergence is greater in the 

 larvae than in the imagines cannot be doubted ; 

 that this distant form-relationship cannot, how- 

 ever, be referred to a very remote common origin, 

 i. e. to a very remote blood -relationship, not only 

 appears from the existence of transition-forms 

 between the two sub-orders, but can be demon- 

 strated here, as in the case of the Hymenoptera, 

 by the embryonic development of the maggot-like 

 larvae. 



Seventeen years ago I showed 8 that the grub- 

 formed larvae of the Muscida in the embryonic 

 state possessed a well-developed head with antennae 

 and three pairs of jaws, but that later in the course 

 of the embryonic development a marked reduc- 



true Diptera, the short-horned (Brachycera), and the long- 

 horned (Nemocera), are not sharply limited ; and I am like- 

 wise well acquainted with the circumstance that there are forms 

 which connect the two larval types. The connecting forms of 

 the imagines do not, however, always coincide with the inter- 

 mediate larval forms, so that there here arises a second and 

 very striking incongruence of morphological relationship which 

 depends only upon the circumstance that the one stage has 

 diverged in form more widely than the other through a greater 

 divergence in the conditions of life. The difficulty is in these 

 cases aggravated because an apparent is added to the true form- 

 relationship through convergence, so that without going into 

 exact details the form and genealogical relationships of the 

 Diptera cannot be distinguished. It would be of great inte- 

 rest for other reasons to make this investigation, and I hope 

 to be able to find leisure for this purpose at some future 

 period. 



8 " Entwicklung der Dipteren." Leipzig, 1864. 



