8 SCIENCE PROGRESS. 



pavonia the tubercles are conspicuous and the knobs acquire 

 an appearance distinct from the general surface of the body 

 in the third or fourth stage. In S. pyri on emergence from 

 the ^^^ the knobs are already coloured and indications exist 

 of the extreme prominence of the tubercles. 



(3) The tactile bristles are least developed in S. spini, 

 most in 6". pyri. 



(4) The cocoon of ►S. spini is simpler than that of S. 

 pavonia. That of S. pyri is the best defended of the three. 



(5) 5. spini is almost sexually monomorphic, though 

 the male is somewhat the smaller. The female is very 

 sluggish, and the antennae of the male are pectinate to a 

 high degree. \xi S. pavonia there is well-marked sexual 

 dimorphism, as regards both size and colouring. The 

 female resembles that of 6". spini in aspect ; it is less 

 sluggish, though not very active. The antennae of the 

 male are less strongly pectinate. ^. pyri again is sexually 

 monomorphic ; the female is a tolerably good flier, and 

 the antennae in the male are less strongly pectinate than 

 in many other species of Saturnia. 



Judging- from the perfect insects alone, we must con- 

 clude, according to Standfuss, that the less specialised 5. 

 spini is older than S. pavonia. We should, however, find 

 it difficult to determine whether ^. pyri arose before or 

 after the divergence of these two. The larval and pupal 

 stages enable us to answer the question. The three species 

 form a progressive series in protective adaptation, 5^. spini 

 always taking the lowest and 5. pyri the highest step in 

 the scale. It must therefore be concluded, on the whole 

 evidence, that S. spini is phylogenetically the oldest, and 

 S. pyri the youngest of the three forms. 



General conclusions with refei^ence to hybridisation be- 

 tween distinct species. As the author elsewhere expresses 

 it, the crossing of two distinct species gives rise to a 

 " Zwischenform " but not to a " Mittelform ". The Mittel- 

 form may, however, exist as a temporary stage in larval 

 growth. This depends on the following principles, which 

 Standfuss considers to be warranted by the above experi- 

 ments with species of Saturnia : — 



