( cxii ) 



before pupation, but after the period at which the larva 

 could spin another, did not alter the normal appearance of 

 line and groove in the lesulting pupa, I was for the moment 

 convinced that acquired characters are hereditary. But 

 fortunately the inquiry did not come to an end at this 

 point. It was observed that the Pierine pupa3 which furnished 

 the material for experiment (P. brassicx or P. rapee) invari- 

 ably suspended themselves either horizoutally or vertically 

 with the head upwards, — never vertically with the head down- 

 wards. Several larvse of P. brassicx had fixed themselves in, 

 the normal vertical position preparatory to pupation, upon a 

 sheet of glass. Before pupation, but after the period at 

 which the larvre could fix themselves afresh or indeed make 

 any attempt to spin, the glass sheet was rotated through half 

 a circle, so that all the larvaj came to be suspended head down- 

 wards. In this position they were compelled to pupate. The 

 condition of the lesulting pupje clearly refuted the hypo- 

 thesis of a mechanically-created groove and thickening, caused 

 by the cutting into and pressure upon the soft yielding cuticle. 

 For in the vertical position with head downwards the pupa 

 slips through the silken loop beyond the position of the groove, 

 so that the pressure has to be borne by an uni)repared part 

 of the cuticular surface. Upon the mechanical hypothesis, 

 we should expect that the fresh surface would gain some 

 measure of resistance from the strain ; but on the contrary 

 the pupa3 were all hopelessly deformed and the imagines, — if 

 indeed they could have emerged at all, — would have been 

 incapable of flight.* It is evident that from the very begin- 

 ning the loop has been accompanied by a sufiicient strengthen- 

 ing of the part of the surface exposed to its pressure as soon 

 as the larval skin was thrown off. 



The silken loop together with the attachment of the pos- 

 terior extremity of the pupa is in all probability the persistent 

 trace of a vanished cocoon, and we may imagine the selective 

 process which made good each step on the road of gradual 



* This experiment Las not been published hitherto. It was however 

 described and the pupre exhibited in the discussion in Section D of the 

 British Association at Manchester, on Monday, Sept. 5, 1887. See Report, 

 p. 755. 



