Vise} 
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 resulting 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 pupe which furnished 
the material for experiment (P. brassice or P. rapx) invari- 
ably suspended themselves either horizontally or vertically 
with the head upwards,—never vertically with the head down- 
wards. Several larve 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 larve could fix themselves afresh or indeed make 
any attempt to spin, the glass sheet was rotated through half 
a circle, so that all the larvee came to be suspended head down- 
wards. In this position they were compelled to pupate. The 
condition of the resulting pup 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 unprepared 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 pup 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 suflicient 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 has not been published hitherto. It was however 
described and the pup exhibited in the discussion in Section D of the 
British Association at Manchester, on Monday, Sept. 5, 1887. See Report, 
p- 755. 
