332 Difficulties of Darwinism. (July, 
that the sting of the latter, though corresponding in funtion 
with that of the wasp or bee, is essentially different. 
From the poison-faculty we pass to a power widely 
distributed among the Articulata, and even found in some 
Mollusca, but totally wanting among vertebrate animals— 
the power of secreting a textile matter and of forming it 
into threads and tissues. Most inse¢ts possess this power 
when in the state of larve, though in very different degrees 
of perfection. Among spiders only it exists in perfection in 
the adult state, and is used as a means of procuring food, 
or, we might say, as a weapon. Are, then, the silk-giving 
insects, arachnides, and mollusks* descendants of one 
common stock, or has it also originated independently in 
different groups? Here, again, it must be remembered that 
the spinning organs are not homologous. The spider emits 
its silk from certain abdominal appendages, whilst all inse¢t 
larvee spin from orifices in the face. The limitations of this 
power are also perplexing. Very useful, or rather necessary 
to many who possess it, it would have been equally useful 
to many who do not. What, then, is the reason that 
Natural Selection has operated just as we find it to have 
done? ‘To many birds, and to certain arboreal mammalia, 
the power of spinning would have been of striking 
advantage. Yet the process which has evolved spinning 
spiders has failed to produce spinning birds and mammals. 
Who can solve this riddle ? 
Yet, again, we find in certain animals the power of phos- 
phorescence—of emitting at will a distinct light. This 
faculty is, I believe, confined to the Articulata, and appears 
there in certain detached groups. It is found amongst 
Coleoptera, as in the well-known glowworm and fire-fly ; 
amongst Homoptera, as in the lantern-fly, and in at least 
one centipede. But would it not have been equally useful 
to many other no¢turnal animals, who might thus have 
attracted each other? If the digression is permissible it is 
curious why nocturnal mammalia are scared and repelled by 
a light, whilst nocturnal inse¢ts are universally attracted. 
What charm can the candle have for the moth? Nocturnal 
birds, also, have been known to dash themselves against the 
windows of lighthouses. 
To return. Either all luminous animals are. descended 
from one common stock, or the power of emitting light has 
been independently elaborated at distinct points, and not 
elsewhere—a repetition of the difficulty we have already 
“Pinna: 
